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Volume 3 Issue 2

In this issue

“Multiple Case Studies” Plus . . .

Vehicle Speed Sensor Calibration Basic Integral Calculus

for Crash Reconstruction

Bus Braking Tests: also Close-Range Photogrammetry & Forensic Photography

With and Without ABS

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Keeping you current in

Crash Data Retrieval

January 26-28, 2009 Houston, Texas Sheraton N. Houston Hotel Subject matter experts from around the world present on the newest topics and research regarding CDR.

The CDR User’s Conference is offered annually in Houston, TX. This is a comprehensive conference to publish Crash Data Retrieval System (CDR) related information in the form of papers, data compendium and the like. It covers case studies, legal issues, legislation, user applications, and future developments. It also provides hands-on interaction through various work shops.

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2 Collision

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Contents Fall 2008 Volume 3, Issue 2

of interest

8

Calendar of Events

10

Professional Societies

4

Letter from Editor

104

New CDR 3.1 Release 56

features 15

Close-Range Photogrammetry as a Routine Crash Reconstruction Tool within the Florida Highway Patrol

22

Vehicle Speed Sensor Calibration

48

Centrifugal Force Is It Real?

56

Basic Integral Calculus for Crash Reconstruction

62

Forensic Photography and Crash Reconstruction, Part 1: The Professional Amateur Expert Witness Photographer

66

School Bus Rollovers and Passenger Compartment Ejection: Solving the Puzzle Using Videoanalytics

70

Bus Breaking Tests: With and Without ABS

78

Hydroplaning: The effect of Water on the Roadway

48

70

case studies 38

Motorcycle vs. Passenger Car

92

2 in 3 Out Revisited

42

Truck to Passenger Car

86

Case Problem - Solution

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from the editor As we bring 2008 to a close, we can look back on the successful completion of the 3rd volume of Collision. What started as something of a labor of necessity has become the leading publication of its kind in the crash reconstruction community. This issue boasts more than a dozen original articles and compilations and not one single news service clipping or reprint of a government report or document. We’re already looking toward Volume 4 which will be built on the proceedings of the 2009 CDR User’s Conference.

While the full spectrum of the User’s Conference hasn’t been finalized at press time for this issue, some of the content anticipated for the gathering in Houston in January includes a review of several aspects of the data collected from a series of Chrysler vehicle crash tests conducted just this past September, a look at the state of EDR data admissibility and recent appellate decisions, and workshops focusing on some of the new developments in CDR technology and applications.

Workshops were one of the highlights of the 2008 User’s Conference and, for 2009, we’re looking at ways to improve on that experience by offering a rotating series of small group workshops focusing on Ford PCM data analysis, Chrysler data analysis and GM RollOver Sensor analysis. The latest release of the CDR program includes a CSV export function for Chrysler vehicle data and, to graph and chart the data, spreadsheets and add-ons like CSVPro put the CDR system user in control of the look of the final output reports.

On that note, the latest release of the CDR system software, version 3.1 boasts new and expanded access to Ford, GM and Chrysler vehicles through model year 2009 and includes many changes in formatting and the quantity and types of data to be recovered. The 09 User’s Conference will again be the preeminent venue for the latest CDR System information. If you’re a CDR System user, you really can’t miss this opportunity to hear from those most closely involved in the ongoing development and evolution of the CDR System.

While I’ve devoted significant space to talking about the User’s Conference in this note, the truth is, it’s the most rapidly developing and broad reaching technological change in the crash reconstruction industry in years. And things will continue to change on that front.

What else will change in the issues to come? For our part at Collision, we’re hoping to be able to include additional crash data from other conferences and testing which will increase the value of your subscription to Collision and, for those of you who are members of one of the participating organizations and get your copy through that membership, that would make the return on your membership all the more meaningful.

We’ll continue to do whatever we can to improve on the value of your subscription or membership subscription to Collision in the coming months and urge you all to contact us and let us know what you want to see in these pages.

Managing Editor 4 Collision

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Collision Publishing LLC PO Box 3383 Kirkland, WA 98083-3383 Toll Free: 800-280-7940 Email: editor@collisionpublishing.com Web: www.collisionpublishing.com ISSN: 1934-8681

Collision Staff Scott Baker Sean Haight Tonya Baker Michelle Jennings

Owner, Managing Editor Editor Advertising Manager Customer Relations

Conference CD Included with this issue of Collision is the 2008 ARC-CSI Crash Conference data DVD. This data DVD includes all the crashtest data collected, crash video footage, digital photos and the Power Point presentations from the presenters.

All rights reserved, © 2006-8 Collision Publishing LLC. The opinions and conclusions expressed in this publication and attached data disk and throughout the articles attributed to specific authors are the opinions and conclusions of the authors noted and not necessarily the editorial staff or anyone else for that matter.

While some articles have been reviewed for content, the accuracy of reprinted models or equations cannot be fully guaranteed. It is the responsibility of the reader to apply critical thinking to an individual review of the content and make their own personal judgements as to its value to them, individually. At the end of the day, facts belong to everybody, any other opinions to us. The distinction is yours to draw...otherwise, the opinions expressed herein are not necessarily those of any employer, not necessarily ours and probably not necessary. Dissenting opinions, discussion or conclusions which may express an adverse position to those expressed herein by specifically cited authors can be addressed in writing by sending an email to the editor at editor@collisionpublishing.com or sending a “regular mail” letter to us at PO Box 3383, Kirkland, WA 98083-3383. By sending correspondence to either our email or snail mail addresses listed in this publication you are agreeing that: (1) we are by definition, “the intended recipient” (2) all information in the email is ours to do with as we see fit and make such financial profit, political mileage, or good joke as it lends itself to and, (3) this overrides any disclaimer or statement of confidentiality that may be included on your original message. The entire physical universe, including this publication and attached data disk, may one day collapse back into an infinitesimally small space.

Should another universe subsequently re-emerge, the existence of this publication and data disk in that universe cannot be guaranteed.

ARC-CSI Crash ARC-CSI Crash Conference CDs Conference CDs

Submit an Article If you would like to submit an article to be published in Collision please email us your article for review or call us toll free 800-280-7940. Email: editor@collisionpublishing.com

Back Issues If you would like purchase back issues of Collision Magazine, please visit www.collisionpublishing.com/backissues.html or call us toll free 800-280-7940.

Adverstising Rates & Information

The Collision Publishing media kit, including demographics, advertising rates, placement and sizes, is available online at: www.collisionpublishing.com

Author Index David Templeton ............................................. pg. 15 Alan Nagel ....................................................... pg. 22 Peter Rast ........................................................ pg. 48 Sean Haight ..................................................... pg. 56 Robert Wyman ................................................ pg. 62 Ray Turner ....................................................... pg. 66 D. Vangi, A. Virga, L. Pinchera ........................ pg. 70 Elvin Aycock .................................................... pg. 78 Gary Lewis ....................................................... pg. 38 Greg Russell ............................................... pg. 42, 92 W.R. “Rusty” Haight ....................................... pg. 42

Advertiser Index 4N6XPRT Systems ........... pg. 108, Inside Back Cover CDR User’s Conference ................................... pg. 1 Collision Publishing ........................................ pg. 9 Collision Safety Institute ................................ pg. 106 Crash Data Group (CDR) .............................. pg. 104 Crash Test Service ............................................ pg. 37 CSV Pro ......................................................... pg. 107 IPTM .............................................................. pg. 85 IST .................................................................. pg. 13 iWitness ........................................................... pg. 21 Kinetic Energy Press ........................................ pg. 36 Laser Technology ............................................ pg. 103 MapScenes .............................................. pg. 2, 51-54 Photomodeler Pro ............................................ pg. 55 REC-TEC ....................................................... pg. 50 Sokkia ........................................... Inside Front Cover Vericom Computers ......................................... pg. 65 Visual Statement ....................................... Back Cover WeCare ...................................................... pg. 13, 37

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From the Desk of the President John H. Meserve NAPARS1@comcast.net

A

s I write this message I have just returned from Atlantic City, New Jersey, where another successful Joint Annual Conference concluded on Friday, October 17, 2009. The subject matter was diverse and well-presented, the weather was wonderful, and the high speed vehicle side impact to pole crash tests went off with nary a hitch, producing great data which was shared with all attendees by conference end. Hats off to our compatriots from the New Jersey Association of Accident Reconstructionists for a job well done!

The completion of that conference means that the torch has been passed to NAPARS for 2009, and your board of directors is off and running with planning for our gala 25th Anniversary celebration at the 2009 Joint Annual Conference which will be held October 6-9 at the Grand Hotel in Ocean City, Maryland. The Ocean City Police Department will be our hosts, and in addition to the five participating organizations (NAPARS, NATARI, MdTAI, NJAAR & NYSTARS) the event will be co-sponsored by the Professional Society of Forensic Mapping (PSFM).

The principal theme of the 2009 conference is Commercial Motor Vehicles; we are working on plans for crash tests involving both straight trucks and truck-tractor semi-trailer combinations, and to also provide an opportunity for every attendee to actually climb into the cab and operate a TT unit (no, not for the crash tests!).

Proposed topics include small vehicle vs. large vehicle momentum considerations, the latest in Electronic Data Recording/Retrieval technology for big rigs, comparison of compliance inspections to post-crash inspections, CMV brake system testing with brakes in and out of compliance for comparison, and more. We are planning on filling every minute of the conference with meaningful training opportunities for accident reconstructionists of all levels. I am especially excited that nearly all of the presenters are members of NAPARS, attesting to the wide expanse of knowledge and experience held by our membership across the county and around the world.

Time is running out on the decision-making process as the implementation process has already begun. But it is not too late for additional input from you, our members. Please let me and/or any member of the Board of Directors know your thoughts and suggestions as soon as possible.

Our 25th anniversary observance during the 2009 conference will be a special time…don’t miss it! Mark your calendars now for October 6-9, 2009. NAPARS has been and will continue to be a leader in the rapidly-evolving reconstruction field and this conference will be further proof of our commitment to maintain that leadership position.

Please feel free to contact me at anytime with comments, questions, suggestions or complaints; the more we all work together the stronger our organization will be in the days and years ahead. Hope to see you at a future NAPARS-sponsored event!

John H. Meserve President, NAPARS 6 Collision

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I

’ve just returned from the Joint Annual Conference hosted by the folks at the New Jersey Association of Accident Reconstructionists in Atlantic City, New Jersey. On behalf of NPARS, I would like to express our appreciation and extend our congratulations to the NJAARS crew for a job well done.

Next year marks the silver anniversary of NAPARS and next year NAPARS will host its anniversary conference in Ocean City, Maryland. It’s been an incredible twenty five years and as I watched this years crash tests in New Jersey I reflected on just how far Accident Reconstruction and Investigation has come in that twenty five year history of NAPARS. When NAPARS was formed a quarter century ago, the state of the art was a handheld calculator….high tech was a calculator that did not need batteries. (you remember the one’s with the little red LED’s….well, maybe you don’t!) “Software,” was a three ring binder which contained formula sheets from IPTM or Northwestern University. Crash Data Retrieval was the statement of the driver or, if you were lucky, a witness.

Now, I venture to say, everyone reading this letter has both a laptop and a desk top, computer both are undoubtedly loaded with much of the latest in reconstruction software. Now we draw with sophisticated mapping and graphics software, gone forever are the days of the “Blue Blitz” and the Northwestern Traffic Template. We measure using lasers and photo optical devices with incredible precision. The 300 foot fiberglass tape or rolling wheel is now all but a thing of the past.

Twenty five years ago there was no internet (at least not like we know it today)….no e-mail….no websites…

There were few cell phones when NAPARS came into existence and, if you had one, it wasn’t going to fit in your pocket…it probably came in its own bag! NAPARS was formed with the idea in mind to help “professionalize” the field of motor vehicle collision reconstruction. At that time there were very few such organizations, now, thankfully, there are many.

So as we look toward next year, NAPARS plans to host its most ambitious Joint Conference to date. We will be concentrating on the topic of commercial vehicle collision investigation. We are planning hands on exhibits and crash tests involving commercial vehicles and heavy trucks. All, with the most sophisticated instrumentation.

Join us as we celebrate twenty five years of service to the Accident Reconstruction community.

One can’t help but wonder what the field of accident reconstruction and investigation will be like when NAPARS turns fifty. What resources will be available in the next quarter century that we can today only dream about.

I am confident of one thing, professional associations like NAPARS, NJAARS, MdATAI, NATARI and NYSTARS and many more will be there to help us keep up to date on the latest in technology. Publications like Collision will continue to be an excellent resource for all of us as we move into the future and will become the historian to remind us from whence we came.

Patrick R. (Pat) Burley Vice President, NAPARS

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The International Association of Accident Reconstruction Specialists is one of the oldest associations dedicated to expanding the knowledge and training of accident reconstructionists.

Formed in 1980, we have members from several different countries as well as many different states across the U.S. We have three levels of membership. Our highest level, Fellow, includes a peer review of a reconstruction project. This peer review feature can be very beneficial to a member seeking expert witness qualification in court. One of the criteria a judge can consider when deciding if a person qualifies as an expert is a peer review of his or her work. We offer opportunities for a member to publish work or research in the field of accident reconstruction. This, also, can be very important when qualifying as an expert witness in court proceedings.

Membership in IAARS has other advantages, including a discount (from IAARS) on a subscription to Collision Magazine, member

ASPACI Inc.

ASPACI is a non-profit society whose members have joined to share in the challenge of analysing the complicated issues of road collision investigation. Web: www.aspaci.org.au ASPACI was formed in 1991 by a group of police and collision reconstructionists who felt there was a need for information sharing between police and associated institutions in the Australia, South East Asia, New Zealand and South Pacific area. Its primary function is to assist in the spreading of knowledge and enhancing expertise in the science. The Association seeks to promote a professional approach to collision investigation and to encourage the highest standard of ethics, integrity and honesty among its members.

My name is Kent Boots, and I am the Chairperson on the California Association of Accident Reconstruction Specialists (CA2RS) board of directors.

I’d like to start by telling you a little about our organization.

CA2RS was formed in April of 1998 with the objective of providing education, training, and resources in the field of accident reconstruction. At the end of our first year we had just over 100 members, and we have grown to over 300 members. Current membership dues are $50 to join the organization and $40 a year to renew your membership.

As part of your paid membership you are allowed to attend all of the quarterly trainings for free. We have three quarterly training sessions a year for both Northern and Southern California, a total of six training sessions. The Northern California training sessions are the first month of each quarter; January, April, and July. The Southern California training sessions are the second month of each 8 Collision

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discounts at our training seminars and others listed on our web page, www.iaars.org.

We have annual training opportunities at different locations across the country. We try to have some sort of actual vehicle testing at each seminar, including full scale vehicle crashes. As a member, you would have access to a vast archive of results from many different and sometimes quite unique tests.

Our 2009 training seminar is scheduled for the week of September 14, 2009 in Boise, Idaho and it will be sponsored by the Idaho State Police. More information can be found on our web site, www.iaars.org If you have any questions about IAARS or the application process, please feel free to contact me or any of the board members listed on our web page.

Sincerely, Dan Lofgren President, IAARS

The early days saw the number of members vary with the base of the association in Victoria, Australia. ASPACI became ASPACI Incorporated in 1993 in Victoria. Now we boast members from all over mainland Australia, New Zealand., United Kingdom, and the United States, as well as parts of Asia.

Membership to ASPACI is open to all persons engaged in collision investigation and its allied fields. The association members include Engineers, Police Officers, Independent Consultants, Scientists, Insurance Corporations, Manufacturers and Safety personnel. ASPACI is not a Trade Association or Political Lobby group, nor is it a Police organisation.

quarter; February, May, and August. We do our best to offer the same training for both the northern and southern training sessions.

In lieu of a 4th quarter training session we have a two-and-a-half-day, conference starting on Thursday and ending on Saturday at noon. There is a fee to attend the conference which typically includes breakfasts and lunches as well as a vendor reception. The conference topics have been “themed” such as Commercial Vehicle Collision Investigation, Bicycle and Other Inline Wheeled Vehicle Collisions, Motorcycle Collisions, and Emergency Vehicle Collision Investigation. We have also had multi-topic conferences such as this year; more conference information is available on the “Events” portion of our web site.

For more information about our organization, training, or conference, please visit our web site at www.ca2rs.com. If you have any questions, please don’t hesitate to contact me via e-mail at chairperson@ca2rs.com.

Kent E. Boots

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5/11/2020 10:12:09 AM

C

Are you missing some issues of COLLISION magazine?

Then why not add to your library and order back issues while they are still available.

ORDER TODAY: Volume 1, Issue 1 Spring 2006 - $59.00 Includes 2006 CDR User’sConference CD

Volume 1, Issue 2 Fall 2006 - $119.00 Includes 2006 ARC-CSI Crash Conference DVD

Volume 2, Issue 1 Spring 2007 - $59.00 Includes 2007 CDR User’s Conference CD

Volume 2, Issue 2 Fall 2007 - $119.00 Includes 2007 ARC-CSI Crash Conference DVD

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Volume 3, Issue 1 Spring 2008 - $59.00 Includes 2008 CDR User’s Conference CD

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Calendar January 2009 1/12/2009 - 1/16/2009 Sacramento, CA Human Factors for Accident Reconstruction Jeff Muttart - www.factualdiagrams.com 1/15/2009 - 1/16/2009 Cherry Hill, NJ Recon Refresher: Time, Distance, and Avoidance SRR Training - www.srrtraining.com

conferences, training, and meetings

3/16/2009 - 3/20/2009 Vallejo, CA Investigation of Motorcycle Crashes IPTM - www.factualdiagrams.com 3/23/2009 - 3/27/2009 Jacksonville, FL Pedestrian/Bicycle Crash Investigation IPTM - www.iptm.org

April 2009

1/26/2009 - 1/29/2009 Houston, TX CDR User’s Conference www.crashconferences.com See Ad on Page 1

4/2/2009 - 4/4/2009 Evanston, IL Basic Physics and Math Workshop Northwestern University nucps.northwestern.edu

1/26/2009 - 1/30/2009 Fort Myers, FL Investigation of Motorcycle Crashes IPTM - www.iptm.org

4/7/2009 - 4/11/2009 Evanston, IL Vehicle Dynamics Northwestern University nucps.northwestern.edu

February 2009 2/2/2009 - 2/6/2009 North Las Vegas, NV Traffic Crash Reconstruction Update NLVPD & IPTM 2/23/2009 - 3/6/2009 Jacksonville, FL Traffic Crash Reconstruction IPTM - www.iptm.org

March 2009 3/2/2009 - 3/6/2009 Jacksonville, FL Investigation of Motorcycle Crashes IPTM - www.iptm.org 3/9/2009 - 3/13/2009 Jacksonville, FL Inspection and Investigation of Commercial Vehicle Crashes IPTM - www.iptm.org 3/9/2009 - 3/13/2009 Tempe, AZ Investigation of Motorcycle Crashes IPTM - www.iptm.org

4/9/2009 - 4/10/2009 Myrtle Beach, SC Recon Refresher: Utility Pole, Rollovers and Occupant ID SRR Training - www.srrtraining.com 4/10/2009 Cary, NC ACTAR Testing for Accredidation www.actar.org 4/14/2009 - 4/25/2009 Traffic Accident Reconstruction I Evanston, IL Northwestern University nucps.northwestern.edu 4/20/2009 - 4/24/2009 Special Problems in Traffic Crash Reconstruction Maitland, FL IPTM - www.iptm.org See Ad on Page 85 4/20/2009 Jacksonville, FL ACTAR Testing for Accredidation www.actar.org

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4/24/2009 - 4/28/2009 San Diego, CA - Cabo San Lucas Mexico Cruise and Reconstruciton Training CAARS - www.ca2rs.com 4/27/2009 - 5/1/2009 Jacksonville, FL Photography for the Traffic Crash Investigator IPTM - www.iptm.org

6/1/2009 - 6/5/2009 Tempe, AZ Inspection and Investigation of Commercial Vehicle Crashes IPTM - www.iptm.org 6/5/2009 Springfield, MO ACTAR Testing for Accredidation www.actar.org

4/28/2009 - 5/2/2009 Evanston, IL Traffic Accident Reconstruction II Northwestern University nucps.northwestern.edu

6/10/2009 - 6/12/2009 Evanston, IL Traffic Accident Reconstruction Refresher Northwestern University nucps.northwestern.edu

May 2009

6/15/2009 - 6/19/2009 Jacksonville, FL Human Factors in Traffic Crash Reconstruction IPTM - www.iptm.org

5/5/2009 - 5/9/2009 Evanston, IL Heavy Vehicle CrashReconstruction Northwestern University nucps.northwestern.edu 5/8/2009 Council Bluffs, IA ACTAR Testing for Accredidation www.actar.org 5/9/2009 Bend, OR ACTAR Testing for Accredidation www.actar.org 5/11/2009 - 5/15/2009 Jacksonville, FL Advanced Pedestrian/Bicycle Crash Investigation IPTM - www.iptm.org 5/11/2009 - 5/22/2009 Jacksonville, FL Traffic Crash Reconstruction IPTM - www.iptm.org

6/23/2009 - 6/27/2009 St. Peters, MO Energy Methods and Damage Analysis in Crash Reconstruction IATAI - www.iatai.org

If you would like to list an event, class, conference or seminar in a future issue of Collision, please send us an email with complete details of the event.

Email: editor@collisionpublishing.com Phone: 800-280-7940

5/14/2009 - 5/18/2009 Evanston, IL Pedestrian/Vehicle Traffic Collision Northwestern University nucps.northwestern.edu 5/19/2009 - 5/20/2009 Philadelphia, PA NATARI Spring Conference www.natari.org

June 2009 6/1/2009 - 6/4/2009 Las Vegas, NV ARC-CSI Crash Conference www.crashconferences.com www.collisionpublishing.com

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Who: You, the CDR System user What: CDR User’s Conference 2009 When: January 26-28, 2009 Where: Sheraton North Houston Hotel at George Bush

Intercontinental Airport, Houston, Texas

Why: The most important information on the latest version of the CDR System including results of full scale Chrysler crash tests, reliability of crash data, admissibility issues, expansion of the CSV function and CDR user workshops

How: Check out http://crashconferences.com for early registration rates, a $200 discount before December 12

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Professional Societies

ARC Network - www.accidentreconstruction.com - Founded in 1998, the ARC Network is the largest Internet portal web site for the industry of accident reconstruction and traffic accident investigation. This organization provides resources and information for all areas of accident reconstruction and traffic accident investigation including research, news, expert discussion, events, products and services, expert witness directory, book store, education services directory, police department directory and a member’s only section that provide valuable online databases and tools to assist in daily accident reconstruction requirements.

ASPACI - www.aspaci.org.au - ASPACI is a non-profit society whose members have joined to share in the challenge of analysing the complicated issues of road collision investigation. ASPACI was formed in 1991 by a group of police and collision reconstructionists who felt there was a need for information sharing between police and associated institutions in the Australia, South East Asia, New Zealand and South Pacific area. Its primary function is to assist in the spreading of knowledge and enhancing expertise in the science. The Association seeks to promote a professional approach to collision investigation and to encourage the highest standard of ethics, integrity and honesty among its members.

CA2RS - www.ca2rs.com - CA2RS was created to provide accident reconstruction training and resources to its members. There is a growng need for more local resources in the accident reconstruction field. This organization has grown to over 260 paid members and are always looking for ways to strengthen and increase their membership.

CATAIR - www.catair.net - The Canadian Association of Technical Accident Investigators and Reconstructionists (CATAIR) was founded in 1984 to meet the growing demand for a professional organization that subscribed to a code of professional conduct. CATAIR membership is open to individuals involved in all aspects of road safety. Current membership is in excess of 250. presently, our membership is comprised of active and retired police officers, professional engineers, automotive engineers, engineering technologists, private consultants and representatives from the insurance industry.

FARO - www.faro-inc.org - FARO, officially known as Forensic Accident Reconstructionists of Oregon, was chartered in June 1994 under the laws of the State of Oregon. Officially designated as a non-profit, mutual benefit corporation, FARO exists as a voluntary professional association of individuals who practice forensic reconstruction in Oregon.

IAARS - www.iaars.org - The International Association of Accident Reconstruction Specialists was started in 1980 as a training and networking resource for members. It is believed we are the oldest such crash reconstruction organization. Our membership consists of Law Enforcement as well as Engineers and PhD’s. We have members from 38 states as well as Canada, England, Singapore, Spain and West Australia. Our association is a little different from many others in that we have a peer review process for membership. This peer review is a specific area of judicial consideration when admitting an expert opinion in court. The annual seminars, held in different locations nationwide, not only provide instruction on specific topics by experts in the field, they provide face-to-face discussions on how other people approach reconstructing a crash. The old saying “two heads are better than one” is definitely true in accident reconstruction. The next seminar will be the Big Event in Houston, TX in September 2006.

IATAI - www.iatai.org - The Illinois Association of Technical Accident Investigators was born after several members of the Illinois State Police completed their training in traffic crash reconstruction. They saw the need for a way to share information, experiences and grow as Accident Investigators. These founding members set four goals for the organization; 1. To provide professional standards for members investigating accidents; 2. To promote the continued expansion of members knowledge in the area of technical accident investigation.; 3. To promote Traffic Safety through accident investigations; and 4. To promote improved capability of each agency through training and idea exchange. Since 1986, IATAI has held annual training conferences throughout the State of Illinois. These conferences serve to bring valuable training from across the country into Illinois. Our membership reaches out to 26 states in both the public and private sector.

MATAI - www.mdatai.org - MATAI continues to be innovative and willing to meet the needs of its membership. Over the years, MATAI has grown into a internationally renowned organization. MATAI has sponsored or co-sponsored numerous conferences that have been attended by fellow investigators from around the world. These conferences have included relevant and at times, spectacular test collisions involving cars, trucks, motorcycles, pedestrians, bicycles, and even a Metrobus.

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MATAI - www.matai.org - Founded in 1988, the Midwest Association of Technical Accident Investigators (MATAI) was formed to provide a professional affiliation for individuals who have a primary interest in the technical aspects of motor vehicle traffic collisions. The Association is dedicated to the exchange of information and ideas to improve investigative techniques and procedures In addition, MATAI provides a communication source between other individuals and affiliations involved in this vital area of public safety. MATAl’s primary objective is to meet the needs of the people directly involved in the initial investigative process.

NAPARS - www.napars.org - The National Association of Professional Accident Reconstruction Specialists is a non-profit organization whose members have joined together to share the challenge of dealing with the complex problems of accident reconstruction and to upgrade and ultimately professionalize the accident reconstruction field. NAPARS is open to all persons who are interested in the fields of traffic accident reconstruction and highway transportation safety. Present membership exceeds 1,100 and includes police officers, engineers, consultants and government safety personnel. NAPARS is also the Premier Industry Partner for Collision Magazine.

NATARI - www.natari.org - The National Association of Traffic Accident Reconstructionists and Investigators (NATARI) was established as a non-profit Pennsylvania corporation in 1984. The goal of NATARI is to provide a source of information to be shared among accident professionals on a national basis. This goal is reflected in the NATARI logo comprised of an outline of the nation, a broken wheel signifying a traffic collision and the words “to solve for safety”.

NJAAR - www.njaar.org - The New Jersey Association of Accident Reconstructionists started in 1991 as a dream of a few New Jersey police officers who wanted to create a network of individuals with interests in the field of accident reconstruction. Its membership has grown to encompass individuals from many states in all fields of accident reconstruction. We have a large membership base that includes many professionals not only from the police community, but the private sector as well.

NYSTARS - www.accidentreconstruction.com/nystars - The mission of NYSTARS is to organize into one body, professionals in the field of ground vehicle collision reconstruction and related activities, who support and maintain a high standard of ethics, integrity, credibility and honor in the field of collision reconstruction; to encourage training programs relating to motor vehicle collision investigation and reconstruction through research and communication of matters of mutual interest; to promote traffic safety and foster a spirit of brotherhood among the organization’s members.

SATAI - www.satai.com - Established as a non-profit corporation in 1982, the Southwestern Association of Technical Accident Investigators, Inc. goal is to encourage and promulgate the development of professional technical accident investigation and highway safety; promote research and development of programs leading to better technical accident investigations; and to promote the development and dissemination of new knowledge in the fields of traffic safety, accident investigation and reconstruction. The Association is organized and operated for scientific and educational purposes.

SOAR - www.accidentreconstruction.com/soar - An organization of accident reconstructionists dedicated to increasing knowledge, enhancing auto engineering, improving traffic safety and the free exchange of information. Membership is open to all people in all aspects of traffic transportation. Membership is comprised of active and retired police officers, professional engineers, automotive engineers and private consultants.

We have members throughout the world.

TAARS - www.accidentreconstruction.com/taars - TAARS was organized in 1986 as an organization dedicated to advancing Research, Knowledge, Education and Safety in the field of accident reconstruction. TAARS is a broad based organization with members from many different fields, including law enforcement, private investigators, engineers and attorneys. TAARS has a current membership of over 220 members.

WATAI - www.accidentreconstruction.com/watai - WATAI was formed in 1979 and is the oldest of the technical accident investigation organizations in the United States. Twenty-five plus years since its inception WATAI remains true to its original mission; “to promote education and research in the field of accident investigation; to encourage interdisciplinary communication between the practitioners of accident investigation; and to cooperate and participate with other organizations dedicated to the science of accident investigations”. To that end WATAI sponsors semi-annual conferences. The Spring Conference usually takes place in May and the Fall Conference generally in the onth of October.

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Close-Range Photogrammetry As a Routine Crash Reconstruction Tool within the Florida Highway Patrol Corporal David A. Templeton, Jr Florida Highway Patrol

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ackground Law enforcement agencies utilize the principles of traffic crash reconstruction for analyzing the dynamics of collision events. Often the police officers reconstruction work is used in trial and other litigation.

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Police have traditionally employed a range of dimensional measurement techniques for traffic crash reconstruct- ion. These include the roll wheel, steel tape and more recently the electronic distance measuring total station. The total station has resulted in more comprehensive scene measurements and better diagrams than traditional methods employed in the past. However, often times, delays in getting the total station to the scene will extend the road closure time. Even with quick response, the time from incident occurrence to completion of both the traditional baseline tape method or the total station, can often be over two hours and as high as four, or in some cases even more. It is clear that no measurement tool is “perfect” in crash reconstruction and that a “tool kit” of devices is best utilized to perform the task. ew Approach In 2002, the Florida Highway Patrol (FHP) and the Florida Department of Transportation (FDOT) agreed on goals of opening Florida highways within 90 minutes from the time the first officer arrived at the crash scene. This new

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course of incident management was identified as the “Open Roads Policy1.”

Goals of the Open Roads Policy include improved officer safety in crash investigation by reducing the amount of time the roadway is closed and to reduce traffic delays, including concerns of secondary crashes.

To support these objectives, FDOT and FHP representatives researched and attended numerous incident management presentations offered by other successful state police agencies around the country that had implemented a technology called “close-range photogrammetry”. During the 2002 FDOT / FHP investigation, the teams consensus was that close-range photogrammetry allows troopers to reduce their on scene time by photographing the crash and later “mapping the crash” using a computer back in the office.

While the Florida Open Roads Policy does not apply to homicide scenes, the FHP has felt the need to quickly process the crash scene and begin opening highway lanes as soon as possible. Regardless of the severity of the crash scene specifics, motorists are generally impatient in ‘long backups’ and expect the roads reopen as quickly as possible. This situation is exacerbated by the need for more accurate scene documentation for the integrity of the investigation, and successful prosecutions. As a result, FHP investigators now

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Image Courtesy of DeChant Consulting Services-DCS Inc2 find themselves having to collect more comprehensive and accurate data in a shorter period of on scene time. Utilizing a digital camera on scene, as a measuring and mapping tool, the FHP has found that photogrammetry is value-added in supporting the Open Roads Policy, resulting is less time on the road verses other measurement methods used in the past. he Photogrammetry Approach The term “photogrammetry” is derived from three Greek words; photos meaning light, gramma meaning to drawn, and metron meaning to measure. It is the science and technology of generating 3D information from 2D measurements, nowadays mostly from digital camera images. The process entails taking pictures (either analog film or digital camera images) from different overlapping view perspectives. These multiple image perspectives are imported into a PC-based photo- grammetry software program where the same entity, using two or more images, are marked i.e., “cross referenced” to each other. The 3D coordinate information is derived via a process

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age-based measurement work is typically exported in the Drawing Interchange Format (DXF) for diagramming use in a CAD drawing program.

As part of the implementation process, the FHP had tried different low-cost commercially sold close-range photogrammetry software systems and ultimately selected the iWitnessTM program www. iwitnessphoto.com. ase Study Although the FHP has successfully demonstrated that photogrammetry can be used on scenes that are tens of meters in overall length, the following example illustrates a twocar crash at night that blocked a busy intersection. The incident involved a violation of right-of-way, where one of the drivers in-

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Markers: courtesy of www.iwitnessphoto.com

The photogrammetric process solves for the camera positions and 3D coordinates, as well as offering the capability of called photogrammetric bundle tri- angulation, where the 3D points are computed automatically without operator intervention2.

The photogrammetric process solves for the camera positions and 3D coordinates. The technology also offers the capability of computing lines and non-contact measured polylines. The operator can add point names to the measured entities, scale the project units, assign a specific coordinate system and ‘mathematically level’ the coordinate system to gravity. The result of the im18 Collision

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rash Scene Mapping Time It took the FHP trooper 13 minutes to acquire the pictures of the crash scene that was 193’ long by 127’ wide. In addition to “imaging time”, it took another 5 minutes to set the photogrammetric markers in the scene before the imaging.

Following the crash scene work, it took 3 hours of office time by one trooper to do the photogrammetry and CAD diagram computer work.

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All night images were taken with the camera being tripod mounted, using time exposures on average of 3 seconds each, which produced good image exposure. Seventeen images were used in the photogrammetry work, on a PC, back in the office. Two hundred and two 3D points were measured in iWitness to an accuracy of ¼” RMS, as verified by scene check distances as well as the program’s auditing tools.

Marking the digital images in iWitness volved was arrested for driving without a valid driver’s license. This case was later resolved in a conviction and the “scene mapping” was important in the litigation.

This crash scene comprised largely of measuring photogrammetric “natural feature points”, being the crosswalks, signage, vehicles, tire markings, and other critical scene evidence.

Photogrammetric “markers” were also used in the measurement process. The markers can be used to “measure evidence”, however, if the officer can photograph discrete points of interest, those features can be 3D measured by natural feature point marking. The markers assist in fast and accurate orientation of the cameras from where the pictures were physically taken from.

The result of the iWitness photo- grammetry The iWitness work was then exported into the Crash Zone (diagramming software) www.cadzone.com where vehicles and other symbols were also added to complete the crash scene diagram. oncluding remarks Close-range photogrammetry has proven to be a value-added tool for the Florida Highway Patrol’s mapping capabilities.

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Although the traditional baseline approach, as well as the total station is still employed within the FHP, it is noteworthy that over 200 FHP troopers have been trained in the iWitness image-based photogrammetry technology, where it is used as an accurate, fast and easy-to-use “tool” in crash scene mapping investigations.

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cknowledgements 1. Mrs. Jennifer Heller, FDOT Traffic Incident Management.

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2. Mr. Lee DeChant, DeChant Consulting Services – DCS Inc.

snapshot of Photogrammetry

PLAN VIEW of 3D measured entities

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Close-range photo-grammetry is non-contact measurement requiring multiple pictures captured at the scene for measuring feature points of interest. The technology is best applied by using a good quality digital camera for fast, accurate and permanent 3D data recording.

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Some Photogrammetry applications in Accident Reconstruction are:

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- Crash scene mapping - Vehicle crush profiling - Homicide scene mapping - Aerial 3D diagramming - Bullet trajectory - Mapping road and surface defects - Using others (bystanders) pictures for extracting critical measurements - Used cooperatively with other measurement technologies for scene diagramming

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Full Page Ad DCS.pdf

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Vehicle Speed Sensor Calibration And Its Potential Effect on Pre-Crash Vehicle Speed Data As Recorded

By An Event Data Recorder

Alan K. Nagel General Motors Training SAIT Polytechnic Calgary AB Canada

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ationale Event Data Recorders have become a crucial piece of evidence, used to assist a reconstructionist in determining the sequence of events and physical kinematics of a vehicular accident. Some Event Data Recorders are capable of reporting the pre-crash vehicle speeds of the vehicle that was observed for a few seconds prior to the deployment event.

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This pre-crash vehicle speed data has been proven to be suitably precise, scientifically reliable and empirically accurate such that it is universally accepted in the reconstruction community as being a useful data point in the calculation of impact velocities, and in momentum analysis of the event. This vehicle speed data is collected by the Airbag Control Module (ACM) from passive observation of “Vehicle Speed” data packets, presented at regular intervals on the in-vehicle network bus. This data is typically sourced by the Powertrain Control Module (PCM) and based on a Vehicle Speed Sensor (VSS) input wired directly to the PCM. However, the perceived accuracy of the transmitted/recorded speeds is directly influenced by the internal software calibration of the Powertrain Control Module (PCM). Calibration of the VSS is necessary as a wide range of vehicle speed sensor designs exist- dependant on the vehicle’s driveline configuration (rear

wheel drive, front wheel drive, 4- wheel drive etc) and type/ model of transmission being used.

As well, optional equipment such as wheel and tire sizing and drive axle gear ratios can also create variables. Therefore the original equipment manufacturers must employ an electronic means to “adjust” the raw vehicle speed sensor signals and correct for any such variables. This is necessary not only for speedometer accuracy, but also for other networked vehicle systems that rely on accurate “Vehicle Speed” data being transmitted over the network bus. Otherwise numerous speed dependant functions such as transmission shifting, variable assist steering, and navigation systems, may perform incorrectly and not function as designed.

These factory calibration values therefore are quite vehicle specific and assumes that items such as tire size (rolling circumference) and axle ratios are as the vehicle was produced at the factory.

If the tire size or drive axle ratios have been physically modified, the vehicle speed sensor calibration would also need to be altered, otherwise the transmitted vehicle speed data (and those stored in the EDR event file) will be INACCURATE. How inaccurate, will depend on the net mathematical differences between the various alterations and the original equipment. This article

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compensate for changes in drive axle ratios and/or tire and wheel sizing. This type of modification is often referred to as “speedometer correction” but affects much more than the driver’s gauge unit. Since it is common for “tuner” vehicles to have larger wheels and tires installed- this adjustment service is often offered by an increasing number of aftermarket speed shops and progressive tire retailers. Generally this type of calibration alteration is not available through OEM dealerships.

The most blatant of these types of modifications would be observed on a four wheel drive truck or SUV where a chassis “lift kit” has been added and significantly larger tires installed. Modifications such as these are popular among 4x4 enthusiasts and promoted in detail within various enthusiast magazines and web-sites. The step-by-step installation of such modifications are even demonstrated on various popular television programs such as TRUCKS!, X-Treme 4x4 and Truck Universe on SPEED and other cable channels.

Figure 1- Typical EDR Pre Crash Data (Courtesy Bosch LLC) attempts to rationalize these variables and provide a suggested protocol to first identify and then if necessary, compensate for these potential errors in the recorded pre-crash vehicle speeds as stored in the EDR event file.

It should be pointed out, that the scope of this article focuses almost exclusively on General Motors brand vehicles, however there may be similar potential variables and issues within EDR stored data for other automotive manufacturers.

Automotive Enthusiasts and Aftermarket Trends Various aftermarket products have become available that allow almost anyone to alter or modify the factory Powertrain Control Module (PCM) calibrations for fuel, ignition timing, transmission shifting as well as diagnostics and emission controls. These products range from simple hand-held products directly targeting the end-user, to industrial grade hardware and software packages designed specifically for professional automotive “tuners” and speed shops.

These products have generally been designed for use with “off-road” high performance or racing applications that permit the utilization of factory electronic systems. However many street driven cars and trucks are receiving these calibration alterations, mostly for potentially improved performance and/or fuel economy.

One final common aftermarket reprogramming capability is the potential to raise or even eliminate the factory imposed engine and/or vehicle speed limiters. These speed limiter functions leverage the electronic fuel injection system to prevent the engine exceeding a preset RPM, and prevent the vehicle top speed from exceeding the DOT speed rating of the factory tires. ehicle Speed Sensors Modern Vehicle Speed Sensors (VSS) are typically either a permanent magnet generator or solid-state digital switching device that creates a varying input voltage signal to an appropriate electronic control module.

The design and placement of the VSS will typically allow it to create a fluctuating AC or DC voltage with an observed frequency directly proportional to the speed of a nearby rotating component usually called the VSS reluctor.

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This rotating reluctor will have a regular and symmetrical ferrous feature machined into it (e.g. gear teeth, splines, notches, gaps or windows) that rotate past or in proximity to the VSS sensing surface in order to create a cyclical and repetitive voltage signal. This signal’s measured frequency will therefore be proportionate to the reluctor/components’ rotational speed.

The reluctor is typically mounted directly to a component in the path of power being transmitted to the drive wheels and is most often a transmission gear, output drive shaft or clutch housing within or connected to the transmission, transaxle, transfer case or drive axle. By observing the signal frequency produced by the VSS the control module can quickly establish the rotational speed of the rotating assembly.

Electronic adjustments to engine fueling and ignition timing are especially necessary when used in combination with extensive engine modifications and/or the installation of “power-adders” such as superchargers, turbochargers or nitrous oxide injection systems.

Another common objective for intentionally altering the factory calibration is to corFigure 2- Hand Held Power Programmer(Courtesy Hypertech) rect the PCM’s vehicle speed sensor calibration in order to 24 Collision

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Figure 3 – Hosts of Truck Universe (Courtesy SPEED Channel)

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Figure 6 - Digital Magneto Resistive VSS

Figure 4- Rear Wheel Drive Variable Reluctance VSS Then, dependant on the sensors location, applicable gear ratios and signal generation characteristics of the specific VSS design (such as the number of reluctor teeth) the observed frequency measurement can be utilized to allow the module to internally calculate the rotational speed of the output drive shaft to the drive wheels. Once the actual rolling circumference of the tires is mathematically applied, the calculated road speed of the vehicle can also be established. It is the results of these internal calculations that are routinely transmitted as a “Vehicle Speed” data packet on the network bus and as stored in the EDR event file.

The vehicle specific “constants” these calculations will be based upon are typically stored within Electrically Erasable Programmable Read Only Memory (EEPROM) within the PCM. ommon Vehicle Speed Sensor Designs Most modern automotive Vehicle Speed Sensors are a simple a Variable Reluctance (VR) signal generator. The sensor itself houses a permanent magnet that has been encapsulated by a wire wound coil with the 2 ends of this coil made available at the VSS electrical connection via a pair of circuits wired to the Powertrain Control Module-PCM (or in some cases the Transmission Control Module- TCM)

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As a ferrous assembly or shaft with symmetrically spaced notches or teeth (sometimes called a “tone-wheel” or reluctor) spins in the vicinity of the VSS, it results in a regular and synchronous fluctuation in the magnetic field within the VSS. These magnetic fluctuations create a mutual inductance effect within the windings of the coil and effectively produce an AC voltage proportional in both amplitude and frequency to the rotational speed of the reluctor.

The developed AC voltage potential and corresponding frequency can be measured across the 2 electrical terminals of the vehicle speed sensor. If the VSS is wired to an electronic control module, it too can be used as a frequency counter.

*Note- in general the developed AC voltage (amplitude) IS NOT used to calculate the speed of the reluctor. This is because of electrical and physical variables that can affect the actual peak-to-peak voltages produced. These variables include electrical noise or potential interference affecting the observed AC voltage levels, as well as the distance or “air gap” between the sensor and reluctor that can vary due to mounting or construction tolerances. Most VR type vehicle speed sensors typically produce from 0 to 100 VAC and 0 to 200 kHz depending on vehicle speed.

Some Vehicle Speed Sensors (VSS) are of a “digital” design utilizing Hall-Effect or Magneto-Resistive (MR) elements. This style of solid-state sensor looks similar in design to other VSS types, and is mounted in a similar fashion and location. However MR type sensors will produce a purely digital signal, in a sense merely a “toggle” of 2 discrete DC voltage levels.

This type of VSS sensors therefore require a DC offset or reference voltage that will be manipulated by the sensing element. In a digital type speed sensor, a solid-state switching element within the sensor will “make-then-break” the electrical continuity in the circuit with each passing “feature” (gear tooth, notch, spline, or window) of the reluctor. Therefore it too will create a measurable signal frequency that is directly proportional to the rotational speed of the shaft or assembly it is monitoring.(0-200 kHz typically) In both cases the actual frequency produced will be a function of the rotational speed of the reluctor assembly AND the number of triggering “features” of the reluctor itself. Therefore at any given speed, the VSS signal frequency is highly dependent on the number of reluctor teeth. actory VSS Calibration The Vehicle Speed Sensor (VSS) as well as other Powertrain control calibration data necessary for normal engine and transmission function, is typically stored within some form of electronically editable memory hardware within the PCM, such as an Electronically Programmable Erasable Read Only Memory (EEPROM) or Flash ROM.

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From an operational perspective, inaccuracies in transmitted Vehicle Speed data will not only result in an inaccurate speedometer, but also early/late automatic transmission shift points, incorrect electronic power steering assist levels, and potentially improper performance of safety systems such as traction control (TCS), and vehicle stability enhancement control systems (VSES).

Therefore the factory VSS calibration will be required to account for ALL of the variables and provide the correct Pulses Per Mile (PPM) settings in order for the PCM to calculate an accurate vehicle speed from the measured VSS frequency.

The primary variables that are accounted for in the VSS calibration generally include:

Figure 5- Typical Vehicle Speed Sensor (RWD trans)

- Location in path of power - Number of VSS reluctor teeth - Tire Sizing (circumference) - Final drive gear ratio (if applicable)

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Figure 9 - Front Wheel Drive Final Drive Assembly be made “adjustable” on vehicles that have different transmissions available, and therefore could be considered a non-adjustable “constant” within some VSS calibrations.

Figure 7 - 4WD Transfer Case Mounted VSS SS Reluctor Variables The primary consideration when deciding on VSS and reluctor design represents a compromise between creating sufficiently high signal frequencies (in order to create the desired resolution especially at lower speeds) and any negative effects of Electro-Magnetic Interference (EMI) produced by the signal generation. This is one of the reasons “digital” type sensors have become more popular in recent years, as the signal amplitude is fixed and can be kept quite low.

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However it is the number of teeth of the reluctor that is of direct influence and therefore must be accounted for within the VSS calibration. The number of reluctor teeth used by most General Motors vehicle speed sensors typically ranges from 10-100 teeth, with the 40-tooth reluctor being most common. This variable may only

ire Sizing Variables Quite obviously, the most influential aspect of tire sizing is the “rolling” circumference of the tire, while at proper operating pressures. This will have direct influence on the VSS calibration as the number of VSS signal cycles per kilometer or mile must be established to calculate vehicle speed.

Changing tire sizes will often result in changes in tire circumference. But it is important to point out that in most modern vehicles; the practical range of available alternate tire sizes is usually quite limited. This is due to the fact that modern vehicle aesthetics and design tend to minimize clearances between the tires, associated body panels, and wheel housings. So to install tires significantly different from the factory equipment is often difficult without extensive modifications. This is particularly true of passenger cars, but less an issue with pick-up trucks.

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That being said however, one must also consider the current state of high-performance, street-racers and “tuner” communities as well- where wheel and tire size changes modifications are considered almost mandatory.

In an attempt to “fill the wells” most of these types of modifications will often (but not always) result in an change in rolling circumference.

Therefore the scope of this article will tend to focus on light duty pickup trucks and SUVs that tend to have not only the physical capacity for significant tire size changes, but also have active enthusiast communities that promote the installation of larger tires.

This is especially true for dual purpose four wheel drive vehicles that are driven on public roadways as well as used for off-roading. It is not unusual, in some parts of the country, to observe a 4x4 pickup equipped with 44” tall tires being driven on the freeway! inal Drive Gear Ratio Variables The effect of final drive reduction gear ratios is generally only a consideration if it takes place AFTER VSS signal production. Therefore in some transaxle equipped vehicles (mostly front wheel drive cars) the final drive gear ratio will NOT create variables that must be ac-

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Figure 8- Tire Sizing 26 Collision

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Vehicle Information System (GMVIS) is provided by General Motors as an on-line resource to most GM dealers/retailers. The GMVIS “Vehicle Build Sheet” might therefore also provide important Regular Production Option (RPO) details relevant to the reconstruction.

For instance, by comparing the rolling circumference differences that exist between the tires observed at investigation to those originally equipped, one can apply a mathematical correction factor to the EDR recorded pre-crash speeds.

Potential Tire Sizing Scenarios Scenario #1(2003 Full Size Sport Utility Vehicle): -Tires on vehicle placard or build sheet: P255-70R/16 which (according to the manufacturer) has a diameter of 30.05 inches.

Figure 10 - Vehicle Certification Placard counted for within the VSS calibration. However, this statement is based on the “typical” FWD VSS placement on the output shaft of the transaxle AFTER the final drive reduction gear set. Therefore, this variable will normally be considered a Rear Wheel Drive (RWD) calibration variable (an exception would be the Chevrolet Corvette and other RWD cars which use an axle shaft mounted VSS.) SS Data and Accident Reconstruction The “Data Limitations” notes included with almost any CDR report (that includes Pre-crash vehicle speed data) has typically stated “accuracy can be affected if the vehicle has had the tire size or final drive axle ratio changed from the factory build specifications”

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-Tires at investigation: P265-75R/16 are measured or listed as having a 31.64 inch diameter. *This difference would result in an error of approx 5.3% in the reported vehicle speed data on the vehicle network.

Because the rolling circumference of the tires was INCREASED by 5.3% in the absence of any correction through PCM reprogramming this vehicle would always be traveling 5.3% FASTER than the speedometer or pre-crash data would indicate.

This is true because the PCM VSS calibration and resulting vehicle speed calculations would assume that 1 revolution of the axle shaft would result in the traveled distance to be equal to the programmed rolling circumference of the drive wheels (94.35 inches) however in this case the actual distance traveled would have been equal to the larger tires circumference or 99.34 inches which represents a 5.3% increase.

The inverse would of course be true should the tires be 5.3% SMALLER than the VSS programming indicates. So… ule #1 Installed tires that are larger than “expected” by the PCM calibration will result in UNDER-REPORTED vehicle speeds by an amount equal to the proportionate difference in the rolling circumference from those installed, to those “expected” by the PCM. And conversely, installed tires that are smaller than the PCM is calibrated for WILL result in OVER-REPORTED vehicle speeds in the Pre-Crash data recorded by the EDR.

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Figure 11 – Service Parts Identification (SPID) Label It is therefore been considered good practice during a reconstruction (where the event data recorder evidence is expected to be utilized) to complete a thorough vehicle inspection- specifically noting observed details such as tire size or other modifications that might impact the reconstruction and/or affect the accuracy of the EDR recorded pre-crash speed data.

The tire sizing can quickly be validated as original equipment by comparing to data available on the Vehicle Certification label/placard on the inside of the driver’s door or b-pillar area.

s This Significant Error?

From this example, it’s easy to surmise that minor changes in tire sizing will NOT result in significant error in the precrash vehicle speed data. For instance, the fact the vehicle referenced in Example #1 was actually travelling at 42 mph instead of the 40 indicated at AE-1 on the CDR report would probably not have a significant effect on the reconstruction. However even this small amount error might certainly make the difference between EDR recorded speeds that might be deemed as legal and under posted speed limits and one that might be classified as “speeding” under the letter of the law. (e.g. Possibly 58 mpg recorded when actually traveling at 55mph in a 55mph zone)

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It’s also important to note that the speedometer in this vehicle would have been in error by the same amount so the driver may not have been even aware of any existing error or transgression. otential Rear Axle Ratio Scenarios Scenario #2 (2004 Chev Silverado ½ ton pickup)

This information as well as other pertinent data such as rear axle ratios and optional equipment can also be determined from the Service Parts ID (SPID) label within the vehicle. (Usually affixed inside the glove box, trunk lid or spare tire cover)

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The vehicle build data such as available from the General Motors

-Actual Rear Axle Ratio= 4.88 : 1 (based on recent work receipts

- Rear axle ratio RPO code on SPID label and GMVIS indicating GU5=3.23 :1

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raised (lower numerically) than the VSS programming indicates. So… ule #2 Installed rear axle gear sets with ratios that are higher numerically (lower gearing) than “expected” by the PCM calibration will result in OVER-REPORTED vehicle speeds by an amount equal to the proportionate difference in the ratios from those installed to those “expected” by the PCM. And conversely, installed gear sets that are lower numerically (higher gearing) than the PCM is calibrated for WILL result in UNDER-REPORTED vehicle speed in the Pre-Crash data recorded.

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ignificant Error?

Almost any accident investigator will certainly agree that the potential for the EDR recorded precrash speeds to be in error by 51% represents significant error, with potentially serious ramifications to both the potential for data misinterpretation AND the perceived reliability of the event file’s pre-crash vehicle speed data in general.

It is important to point out, that while this example represents an extreme case of axle ratio modification, even a change to/from 3.23 to 3.72 gearing (a common change to improve towing capability or fuel economy) will result in a 15.2% error in the event the PCM’s VSS calibration WAS NOT altered to correct for the changes.

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Figure 12 - Vehicle Build Sheet (typical) from GMVIS

*In most cases, on Front Wheel Drive (FWD) vehicles (or any vehicle where the VSS is mounted AFTER final drive reduction) no VSS correction will be required due to drive axle ratio. Therefore any changes to the final drive ratio will NOT affect EDR recorded pre-crash vehicle speeds. ther Potential Programming Issues Another potential impact of vehicle programming that is related to the VSS and its calibration is the potential for a modification being made to the “top-speed limiters” within the PCM calibration.

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In general, many manufacturers will program into their calibration a top-speed value that will prevent the vehicle from exceeding the DOT speed rating of the factory equipped tires.

Figure 13 - Online Gear Set Ad (Courtesy MY4BY.COM) found in glove box and eventually confirmed via driven/drive tooth count) *Assume NO PCM Re-Programming performed *This difference would result in an error of approximately 51 % in the reported vehicle speed data on the vehicle network.

Because the rear axle ratio gearing was LOWERED (higher numerically) in the absence of any correction through PCM reprogramming this vehicle would always be traveling 51% SLOWER than the speedometer or pre-crash data would indicate.

This is true because the actual rear axle gearing of 4.88 represents a 51% increase to the programmed gearing of 3.23 to 1 Therefore the actual distance travelled, with each rotation of the ring gear will 51% less than anticipated by the PCM.

The Powertrain Control Module (PCM) utilizes the Vehicle Speed Sensor (VSS) to calculate the current speed of the vehicle- based on its VSS calibration programmed into the EPROM memory within the PCM. When the programmed “top-speed” value is reached the PCM will cease the electrical triggering of the fuel injectors. With the engine power immediately dropping due to this the lack of fueling, the vehicle will quickly slow until the fuel injectors are reinstated, usually after a 2-3 mph drop.

This intentional hysteresis allows these calibrated adjustments to act as a top-speed governor, mitigating the potential for tire failure due to speed induced factors. It is important to point out that this calibration factor generally IS NOT something that can be routinely altered by OEM dealership technicians, even at customer request.

The inverse would of course be true should the rear axle gearing be 28 Collision

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Subsequently the dealership reprogramming tools by themselves are not of much use as an investigative tool, with respect to the vehicle speed sensor calibration or other useful aspects within the calibration, such as the top-speed limiter.

It is important to differentiate the aftermarket processes of “editing” or “altering” the factory calibrations and the absolute need for dealership (and aftermarket) automotive repair facilities to have “Service Programming” capabilities.

Service Programming generally allows the technician to: a) Program a replacement control module (ECM, PCM, BCM etc) with the correct calibration “package” for that particular vehicle, and allow it to be put into an operational state, and containing all of the necessary calibration details to permit the module to correctly function in the car or truck.

Figure 14 - Fuel Injector Schematic (Typical) EM Programming Processes As a general rule, the OEM dealership technicians are quite limited in their ability to alter the “details” of the factory calibration. In order to insure the vehicles being serviced are correctly maintained, and meet current emission standards, they have limited access to calibration files themselves but not able to directly edit individual aspects of the calibration. A piece of OEM software (such as GM TIS) allows the ability to electronically compare the calibration ID numbers currently stored in the PCM against an on-line or locally served database containing the most recent updates. These files are organized in the database using each vehicle’s unique Vehicle Identification Number (VIN).

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If an update is available, they can perform the “reflash”- but cannot actually select which software files are installed, nor can they edit any of the actual calibration details locked within the calibration “package” for any given module. However, there MAY be certain optional settings or adjustments may still need to be performed, after reprogramming the PCM. These adjustments are usually performed through the use of a hand-held diagnostic scan tool such as the TECH2.

b) Alter the module’s calibration with an “updated” calibration file or files that will improve general functionality or behavior such that it provides improved satisfaction with the vehicle’s operation. c) Alter the module’s calibration with an updated calibration file or files (created by the OEM) that corrects for a factory authorized reconfiguration of the vehicle. This might be necessary due to the installation of a factory accessory or kit installed by the dealer. In such case however, the factory provides on-line access to the necessary calibration files using a special security process.

These 3 examples of service programming, it is expected these functions be performed ONLY with calibration files provided by the OEM and ONLY with equipment approved through various SAE and EPA directives. The ability to adjust or modify the actual calibration files themselves is generally considered an “unapproved” procedure.

Additional information regarding “Service Programming” capabilities and practices is available from the National Automotive Service Task Force- www.nastf.org and GM Technical Service Bulletins: 73-65-13A, 02-06-04-057A, and 04-03-10-007E Potential Investigative Techniques The impact of tire or rear axle ratio changes will obviously depend on IF the particular PCM had been re-programmed to correctly adjust the VSS calibration to compensate.

So the difficulty remains on exactly HOW to determine if the VSS calibration is factory stock, or has modified by some sort of aftermarket equipment. Then, when necessary, establish by what factor these adjustments or lack thereof would have had on the VSS data packets and the EDR reported pre-crash vehicle speeds.

Should one of these modified vehicles become involved in a vehicular incident, the accuracy of the pre-crash data might then be scrutinized against the potential that observed modifications to the tires (or other gear ratio modifications) MAY or MAY NOT have been accounted for in the PCM VSS calibration.

Obviously IF it can be determined that the PCM calibration HAD been correctly altered to reflect any change in tire size and/or rear axle ratio, THEN the pre-crash vehicle speed data values stored in the event file, should be suitably correct and accurate as has been previously assumed.

IF the calibration HAD NOT been correctly compensated for, then the EDR stored vehicle speed pre-crash data might need to be manually “adjusted” by an amount equal to the mathematical difference the unaccounted for modifications represent.

Figure 15 - TECH-2 Scan Tool

Exactly HOW to establish whether or not the factory PCM calibration had been altered can be accomplished in a number of ways.

How and if this may be accomplished will depend on the condition of the vehicle and the Powertrain Control Module. www.collisionpublishing.com

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EFI Live Limited P.O. Box 54-160, Bucklands Beach 1730 Auckland, New Zealand Therefore it is apparent that an investigative technique using tools such as these would need to be developed in order to determine IF the exiting Vehicle Speed Sensor calibration within the PCM correctly matches the tire and drive axle equipment being used.

Figure 16 - HP Tuners VCM Suite (Courtesy HP Tuners LLC) But in general the suggested techniques will require access to the same sort of enhanced hardware/software packages used to alter the factory calibration files in the first place. By downloading the existing PCM calibration file, the VSS calibration can be analyzed to determine IF the values are correct. The values might also be compared to a stock calibration, in order to establish if there have been modifications from factory.

One of the more popular of these PCM editing packages available goes by the name HP Tuners VCM Suite and is marketed in the USA and Canada by: HP Tuners LLC P.O. Box 801057 Valencia, Ca 91380-1057 The package includes hardware that interfaces to the PCM via the standard OBDII (SAE J1962) DLC under-dash connection. Once downloaded a set of software tools simplifies the file editing process, prior to uploading the modified files, back into the PCM.

Due to the large number of vehicles that HP Tuners VCM Suite supports it is currently the recommended tool for this purpose. For an up-to-date list of supported vehicles visit: http://www.hptuners.com/products/vcmsuite_vehicles.php The supported HPT vehicle list also closely coincides with many of those currently supported by the Bosch Crash Data Retrieval (CDR) equipment, currently the tool of choice for downloading the contents of the event file from the Airbag Control Module (ACM). Therefore, in the context of this article, the use of HP Tuners VCM Suite (henceforth referred to as “HPT”) will be assumed.

Using HP Tuner VCM Suite (HPT) to Verify VSS Calibration Accuracy This may represent the most expedient solution to confirm the calibration (in either stock or modified form) was correct for the equipment present at the time of any EDR event file creation. However it will require that sufficient electrical system integrity exists such that Data Link Connector (DLC) communication with the PCM can be established, so the calibration files can be downloaded using the HPT cable and software. It also requires the investigator to

Since it is tools such as the HP Tuners VCM Suite that are routinely used to modify the PCM calibration these packages can also potentially be used as a forensics tool allowing the investigator to download, translate then analyze and/or compare the calibration files.

A similar package that may be used in a near identical fashion would be a product known as EFI-Live FlashScan marketed by:

Figure 17 - EFI-Live Flashscan (Courtesy EFI Live Ltd) 30 Collision

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measure the tire diameter or circumference and confirm rear axle gearing (on rear wheel drive). This will provide the necessary data to mathematically calculate the expected VSS calibration values, then compare these results to the VSS calibration values downloaded from the PCM. Example #1 (2006 GMC Canyon pickup) Tires at inspection: Goodyear Wrangler MT/R 30x9.50/15LT with measured circumference of 94.6” (diameter 30.11” ) Stock tire indicated by RPO code QCE P205/75R15 (27.1” diameter / 85.13” circumference @1010tire) Axle Ratio at inspection: RPO code on SPID label is GT4 (3.73) and confirmed via manual driveshaft rotation technique. (approx 3.75 rotations of driveshaft to complete 1 revolution of rear wheels) HPT VSS Calibration Data: (in this case includes HPT Tire/Ratio Calculator) As shown above, the HPT collected VSS calibration appears to have the correct axle ratio, but according to the HPT VSS calculator (not available on all PCM downloads) the tires programmed into the PCM should have a diameter of 27.42 inches (stock values) while the tires present on the subject vehicle were measured to be 30.11 inches.

Applying Rule #1 this difference of 2.69 inches represents a VSS error of 9.8%, therefore this truck would have been travelling 9.8% faster than the pre-crash event file data indicates.

Seconds Before AE Vehicle Speed (MPH) -5 44 -4 44 -3 44 -2 44 -1 43

“Adjusted” Vehicle Speed 48 48 48 48 47

Figure 19 – Pre-Crash Excerpts from CDR/EDR Report Example #2 (2006 GMC Chevrolet Cobalt LT) 4-speed automatic transmission (RPO MX0) *Car has been heavily modified with body kits, a turbo-charger, headers and the suspension lowered.

Tires at inspection: Bridgestone Potenza P215/45ZR18 with measured circumference of 76.0 inches (diameter 24.2 inches ) Stock tire: P195/60R15 on vehicle certification placard and indicated by RPO code QTU –on SPID (25.60 inches according to dealer tire specifications) Axle Ratio at inspection: Not Applicable

Figure 20 - Screenshot HP Tuners- Speedometer/VSS Front: P255/45R19 Rear: P295/40R20 with measured rear tire circumference of 89.25 inches (diameter 28.4 inches ) Confirmed as stock Goodyear tire indicated by RPO code QMX (Goodyear spec for rear tire as 28.38”) Axle Ratio at inspection: RPO code on SPID label is GT4 (3.73:1) however manual driveshaft rotation technique yields approx 4.5 rotations of driveshaft to complete 1 revolution of rear wheels (eventually confirmed via driven/drive tooth count as 4.56:1) Downloaded HPT VSS Calibration Data: In this case the HPT data indicates values that were later confirmed as 100% stock values. However based on an aftermarket cover on the rear differential (and a 12.95 ET “dial-in” painted on the windshield) the rear gear set teeth were counted and the truck was found to have a 4.56 :1 gear set installed for drag racing. *Turns out this is an extremely popular SSR modification based on the following SSR forum thread:

Downloaded HPT VSS Calibration Data: In Example#2 the HPT Tire & Ratio Calculator indicates the tire diameter is at the factory settings (25.70 inches) In this case when we apply Rule #1 we find the event file pre-crash vehicle speed at AE-1 of 64 mph would have been OVER reported by a factor of 6.2%. Therefore the “adjusted” pre-crash vehicle speed at AE-1 would have actually been 60.0 mph.

http://www.ssrfanatic.com/forum/f5/4-56-gear-club-whos-21032/ In any event in this case we must apply Rule #2 the EDR precrash vehicle speeds would have been OVER-reported by a factor of 22.3%

*Note* When a vehicle has a VSS sensor placement that takes place AFTER the final drive gear reduction, (usually FWD) the trans and final drive ratios typically are adjusted to read 1.00 Example #3 (2005 Chevrolet SSR Sport Coupe) Tires at inspection: Goodyear Eagle www.collisionpublishing.com

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Figure 21- Screenshot HP Tuners- Speedometer/VSS

Figure 23- Screenshot HP Tuners- Speedometer/VSS

Seconds Before AE

The FWD formula to apply will be: Tire circumference (inches) = VSS pulses per rev × 63360 inches per mile VSS pulses per mile

Vehicle Speed “Adjusted” Vehicle Speed (MPH)

-5

74

58

-4

73

56

-3

71

55

-2

71

55

-1

70

54

The RWD formula to apply will be: Tire circumference (inches) = VSS pulses per rev × 63360 inches per mile VSS pulses per mile ÷ rear axle ratio

Figure 22 - Pre-Crash Excerpts from CDR/EDR Report In many cases the HPT Vehicle Speed Sensor table will NOT include the Tire/Ratio calculator. In those instances the accuracy of the observed VSS calibration values will need to be calculated using values included in the HPT VSS table and compared to the inspected values for tire circumference and when necessary rear axle ratio.

When using this technique the most important values provided by the HPT collected VSS calibration data will be the VSS Pulses Per Mile (PPM) and the VSS Pulses Per Revolution (P/Rev) values.

(And rear axle ratio if RWD) *Refer to HPT Definitions in the Appendix for more information on these VSS Calibration Scalars.

The following formula will allow the investigator to calculate the expected/calibrated tire size from the scalar values provided in the HPT download.

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*Note* These formula are derivatives of the standard Pulses Per Mile equation: Pulses Per Mile = VSS pulses per rev × 63360 inches per mile tire circumference (inches) × 1/rear axle ratio* *if applicable Example #4 (2002 GMC Sierra 2500HD 4WD) Tires at inspection: BF Goodrich Mud Terrain T/A 35x12.5R/16.5 with measured circumference of 109.30 inches (diameter 34.8 inches ) Stock tire indicated by RPO code QIZ -P245/75R16 Axle Ratio at inspection: RPO code on SPID label is GT5 (4.10:1) and confirmed via drive/drive tooth count Downloaded HPT VSS Calibration Data: Applying the RWD formula above we calculate the tire circumference as “expected” by the VSS calibration as 109.3 inches (34.8” in diameter)

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109.30 inches = 40 X 63360 95066 ÷ 4.10 Therefore it appears that despite this vehicle being equipped with tires that were significantly larger than factory, the PCMs VSS calibration must have been adjusted to correct for them. Any PreCrash Vehicle Speed data should therefore also be accurate to expected tolerances.

Had the stock values for tire sizing been in place within the VSS calibration (stock tire has a 95.5 inch circumference) the event file pre-crash vehicle speeds would have been UNDER-reported by 14.5% once we apply Rule #1. dditional Independent Testing In order to corroborate the information included in this article, live crash tests on the affects of VSS programming issues were performed at the 2007 ARC-CSI Crash Conference in June 2007.

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Specifically Crash #2 (Malibu vs. Aspire) was used to demonstrate the EDR recorded effects of incorrect VSS calibration. Prior to the test, the bullet vehicle (Malibu) was programmed using HP Tuners VCM Suite with an incorrect VSS calibration equating to having tires 20% larger than actual equipment.

After the crash, the EDR downloaded pre-crash vehicle speeds indicated speeds 20% higher than those observed using independent radar and timers clocking the bullet car. (This conforms to Rule #1) *For more information refer to the 2007 ARC-CSI Crash Conference DVD

Figure 24 Screenshot HP Tuners- Speed Limiter

omparing Collected Calibration Files to Stock Programming This technique might prove useful, in the event the modified calibration values of concern cannot be calculated or determined via the previously discussed methods. These may include the vehicle speed limiter values or various engine control variables such as maximum turbo/supercharger boost levels or other modifications to items such as the engine torque management settings that might have contributed in some way to the event.

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The process can generally be completed through both destructive and non-destructive techniques.

The basic premise to the “destructive” technique would be to download the PCM calibration data via HP Tuner VCM Suite.

Then have the PCM reprogrammed using the factory equipment using calibration files specific to that vehicle’s VIN. The stock calibration can then be downloaded off the PCM, and a detailed comparison can be made from within the HPT software suite. HPT software features allow easy visual identification and comparison of calibration details by highlighting ANY differences between the 2 files. It can also create a detailed textual report of any differences.

Example #5 - 2003 GMC Sonoma pick-up Heavily modified with V8 conversion Apparent high speed tire failure HPT Downloaded Speed Limiter Values: Stock Speed Limiter Settings: At Time of Investigation:

Figure 25 - Screenshot HP Tuners- Speed Limiter

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Example #6 – 2005 Cobalt SS Supercharged 2.0L 4cyl (RPO LSJ) w/ factory Eaton supercharger Visually observed modifications: Cold Air Intake, header and large 3” exhaust system (catalytic converter removed) 2.6” supercharger drive pulley (stock is 3.4”) Dyno-sheets and tuning receipts found in glove box indicated 325WHP and 22 psi boost (stock is 12psi maximum) Stock turbo boost map:

Figure 26-Screenshot HP Tuners Supercharger Max Boost At Time of Investigation:

Figure 29 - Screenshot HP Tuners - Fuel Cutoff

Figure 27-Screenshot HP Tuners Supercharger Max Boost The 17 year old driver of this vehicle lost control after the engine “grenaded” in a street race and ended up “parked” against a lamp pole at the side of the roadway. Just missing a group of “spectators” S e c o n d s Vehicle Speed Engine Speed Percent Throttle Before AE (MPH) (RPM) -5

18

3472

100

-4

24

4845

100

-3

34

6794

100

-2

39

7810

100

-1

33

4466

0

Figure 28 – Pre-Crash Excerpt from CDR/EDR Report Failure analysis of the 2.0 liter ECOTEC engine indicated that the engine speed (RPM) exceeded the return spring rate limits of the valve springs. Valve float (hysteresis) occurred resulting in valve At Time of Investigation: keeper failure and eventual piston to valve contact Figure 30- Screenshot HP Tuners - Fuel Cutoff The basic “non-destructive” technique is quite similar but preserves the original PCM and programming. In order accomplish this, one would have to purchase an exemplar PCM that represents the “service” part number for the subject PCM and have it programmed to factory specifications. Then by downloading both PCMs with HP Tuners, a comparison of calibration settings can be made- without loss of the original state of the subject PCM AE J2534 Pass-Through Programming This SAE Recommended Practice provides the framework to allow reprogramming software applications from all vehicle manufacturers the flexibility to work with multiple vehicle data link interface tools from multiple tool suppliers.

This system enables each vehicle manufacturer to control the programming sequence for electronic control units (ECUs) in their

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vehicles, but allows a single set of programming hardware and vehicle interface to be used to program modules for all vehicle manufacturers.

Therefore as more J2534 compliant programming packages are released, PCM programming will become easier to access. Investigators may even choose purchase these tools themselves. The GM dealer J2534 tool is known as the Multiple Diagnostic Interface EL-47995*.

Bosch markets a near identical J2534 compliant tool called the “Flasher” and is PN# 01002902* (MSRP $1675.00) *Requires OE-supplied calibration files available from their respective websites and/or CD subscription.

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cuits to the PCM, allowing it to be programmed using the same equipment and software normally used. efer to GM Technical Service Bulletin TSB# 73-65-13A for a list of NAO GM dealers that have a J41207 OBPA and may be able to program a new ECM based on the VIN provided for the subject vehicle.

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The GM OBPA tool may also be purchased from Bosch as PN 01002760 (MSRP $1525.00)

Figure 31 – LSJ Piston Damage due to “Dropped” Valve General Motors (GM) Service Information and Technical Information To enable Independent Service Centers to be competitive in the automotive aftermarket, AC Delco offers GM Service Information (SI) and Technical Information System (TIS) subscriptions via the Web. For more info visit: http://acdelcotechconnect.com/html/tss_tech_esi.jsp

onclusion The accuracy of the pre-crash vehicle speed values recorded by the Event Data Recorder (EDR) has been studied, tested and documented to be sufficiently accurate and reliable for accident reconstruction. However this assumed accuracy is highly dependent on having the correct internal VSS calibration settings within the Powertrain Control Module (PCM) that accurately reflects the correct wheel circumference, final drive ratios, and other variable factors related to Vehicle Speed Sensor (VSS) design and implementation.

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In the past, the assumption was the factory Vehicle Speed Sensor calibration would be present and the recorded Pre-Crash vehicle speeds would be suitably accurate. However, in recent years the emergence of various aftermarket reprogramming tools is making it possible for the end-user to make direct alterations to the vehicle speed calibration within the PCM. Through a highly plausible sequence of events, these alterations or inaccuracies within the vehicle speed calibration itself have the potential to be reflected in EDR recorded Pre-Crash Vehicle Speed data and introduce significant error. The same equipment might also be used to alter the maximum vehicle speed that the vehicle can achieve, that may allow the vehicle to exceed the tire’s DOT maximum speed ratings.

Figure 32- GM Electronic Resources ehicle Too Damaged?

Often the vehicle electrical system is damaged during a collision and reliable HPT calibration downloading or other programming procedures cannot be performed reliably. Downloading and programming the subject PCM does not necessarily require the subject vehicle.

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If the PCM is undamaged, in many cases it can be removed from the vehicle and the download or programming can often be performed as a “bench” procedure. This is accomplished through the use of equipment such as the Kent Moore J41207-C Off-Board Programming Adapter (OBPA).

This article was meant to document research into this phenomenon, provide real world scenarios, and attempt establish potential protocols that could be used to detect and if necessary compensate for potential tampering with vehicle speed sensor calibration or modifications that would affect its accuracy.

Only through sustained research and training in this phenomenon within the CDR User community, can we continue to insure the modern accident investigator can continue to rely on the Event Data Recorder data as accurate and provide scientifically truthful evidence.

See Appendix on attached data disk...

Figure 33 – Kent Moore Off Board Programming Adapter This OBPA adapter is equipped with an SAE standard OBDII DLC connector (SAE J1962) and exchangeable PCM connectors that will provide the necessary power, grounding and data line cirwww.collisionpublishing.com

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Motorcycle vs Passenger Car Gary Lewis

Case Study

n January, 2008, there was a motorcycle vs car collision.

Being cold outside it was unusual to get called for a motorcycle collision, but the weather had warmed to a balmy 50 degrees. This collision occurred in a residential area on a narrow, unlined asphalt roadway. The speed limit was 25 miles per hour. At the scene, there was a 2005 Lincoln Town Car and a 2005 Harley Davidson motorcycle.

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The Lincoln had damage to the left front corner, specifically the bumper, tie rod, and wheel assembly. There was more, less significant, contact damage on the left front fender, behind the wheel, and the driver’s door. All the damage on the Lincoln was low, with none extending onto the hood, windshield, or other areas.

An examination of the roadway revealed several gouge/scrape marks and some tire/skid marks. A long, curved skid mark was left by the motorcycle before impact with the Lincoln. The skid mark was approximately 60 feet long. The motorcycle was traveling up a 6% grade before the collision. The skid mark started near the center of the roadway and led onto the Lincoln’s half of the road. At the end of the skid mark there were several scrapes and gouges that were to the right side of, and beyond, the skid mark. The motorcycle went down on its right side before impact with the Lincoln. After striking the Lincoln, the motorcycle spun in the roadway, as evidenced by circular scrapes, and came to rest in the roadway, just past the area of impact.

The motorcycle was found lying in the roadway, to the rear of the Lincoln. The motorcycle had scratches on the right side. The rear fender was bent upward and there was evidence of contact to the underside of the motorcycle frame. The rear shocks were broken.

There was no significant damage to the front forks of the motorcycle.

The Lincoln’s left front wheel was damaged and wheel was bent to the side after the impact. This wheel slid after impact, leaving a “skid” mark, post-impact. The right side of the Lincoln left scratches on the roadway, caused by the loose gravel along the edge of the roadway. The driver of the Lincoln stated that he applied brakes at about the time of impact, and stayed on the brakes until the vehicle stopped.

The roadway surface was asphalt. It is a residential street with single family homes on each side. There are no lane lines or edge lines. The roadway is approximately 20 feet wide. There was some loose debris along the roadway edges. The debris was salt and sand that was deposited on the roadway about a week earlier, during a snow storm. The roadway was slightly curved and on a grade. The collision occurred about 250 feet from an intersection.

Witnesses stated that the motorcycle involved in this collision had been riding with two other motorcycles and a car that entire afternoon. According to the witnesses, the three motorcycles and the car had been “racing around the neighborhood”. The car that had been traveling with the motorcycles was parked along the side of the roadway, partially in the roadway, adjacent to where the motorcycle’s skid mark began. The driver of the Lincoln stated that two motorcycles passed by him in the opposite direction and that they were “flying down the road”. According to the driver of the Lincoln, the third motorcycle passed the other car and went down

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motorcycle did not have ABS. The mark then became solid and uniform for the remainder of the skid.

The motorcycle’s pre-impact speed loss was calculated and found to be approximately 27 to 30 mph. That is not its initial speed, but only that speed lost prior to impact with the Lincoln. Obviously, the motorcycle was exceeding the posted 25 mph speed limit of the roadway. Coincidentally, the Lincoln’s post-impact speed loss was found to be from 27 to 30 miles per hour if he applied the brakes fully. The loose gravel along the roadway showed some areas of scratching, but not for the entire distance. I don’t think that the driver of the Lincoln “locked the brakes” after the collision, but rather, that he hit the brakes and came to a quick, but controlled stop. Therefore, I believe that he was traveling at, or near, the posted 25 mph speed limit.

The collision between the motorcycle and the Lincoln occurred on the Lincoln’s side of the roadway, even though the center of the roadway is unmarked. The collision occurred approximately 13 feet from the roadway edge on the motorcycle’s side, and approximately 6 ½ feet from the roadway edge on the Lincoln’s side.

Although the motorcycle’s initial speed was over the posted speed limit for the roadway, the roadway characteristics were such that the motorcyclist should have been able to pass the stopped car, if, in fact, it was stopped along the roadway when the collision occurred, without losing control. If the car was moving, the operator might have panicked and applied the rear brake, causing the motorcycle to cross toward the outside of the curve and into the oncoming Lincoln. Whether, or not, the car was stopped along the roadway, or traveling on the roadway in the same direction as the motorcycle, seemed to be a rather important factor in this collision.

If the vehicle was moving in the same direction as the motorcycle, then the motorcyclist was passing when unsafe to do so. If the vehicle was parked, then it would appear that the motorcyclist was greatly exceeding the posted speed limit and panicked when he saw the oncoming Lincoln. Either way, the motorcyclist was clearly at fault for this collision. The motorcycle operator was killed in this collision, and, therefore, unable to say what he was doing at the time of the collision.

A few days after this collision occurred I went to the repair shop where the motorcycle was being stored. When I arrived, one of the mechanics asked if I knew the history of this bike. I told him that I had noticed all the modifications done to the motorcycle, but was unaware of its “history”. The mechanic told me that the operator of the motorcycle had done a lost of customizing to the bike, to include lowering the rear end. The mechanic then handed me a broken shock. He said, “This is the shock that I replaced last month.” I looked at the end of the broken shock and noticed that it was a “clean break”, in that the piston wasn’t bent at all, it looked as though it was just cut off. I then looked at the motorcycle’s broken rear shocks, yes; both rear shocks, and saw similar breaks.

on the roadway before striking his vehicle. The driver of the other vehicle stated that he was traveling behind the motorcycles the entire time and that he stopped at that location when he saw the collision occur.

Looking at the skid mark left by the motorcycle, pre-impact, I was confident that the mark was left by the rear tire of the motorcycle.

The skid mark began just to the rear of the parked vehicle, continued across the center of the roadway, and ended where the scrapes and gouges began. The roadway curves to the right in the direction that the motorcycle was traveling. It appeared that the motorcyclist attempted to pass the stopped vehicle, saw the Lincoln coming toward him, and locked the rear brake in an attempt to avoid the oncoming Lincoln. At the scene, I noticed that the skid mark had a slightly unusual appearance. Near the beginning of the mark it appeared to get dark, then slightly lighter, then darker, again, several times. The mark almost looked like ABS braking, although the 40 Collision

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Upon looking closely at the broken ends of the piston, I noticed that there appeared to be a ring of metal around an inner piece. It looked almost like a soldered joint. Speaking with the mechanic, I found that the shocks had been cut down to lower the back of the bike approximately 2 inches. The piston of the shock was shortened by 2 inches, and then the piston was ground down (in diameter) and welded. The weld was then ground down to match the original diameter of the piston. According to the mechanic, the shock that had broken the previous month had broken when the owner was making a U-turn, to the left, in a parking lot and the left rear shock broke, causing the operator to lose control and fall onto his left side.

Now there was something else to consider in this collision. Was this collision the result of speeding and improper braking, or did the broken shocks cause the rider to lose control. I looked under the rear fender of the motorcycle. Sticking through the bottom of the fender was a ½” bolt. The bolt held on the back of the motorcycle’s seat. There was black residue around the bolt. I looked at

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the rear tire and noticed a shallow groove around the circumference of the tire. The groove appeared to be very similar in width and depth to the other tread patterns on the tire. The groove was near the center, but just slightly off the centerline of the tire and was continuous around its circumference. Manipulating the wheel and fender I was able to see that the groove around the tire was made by the bolt. Now, I knew why the fender was bent upward.

It was now clear that, even though the motorcycle was speeding before the collision, its speed was not the primary cause of this collision. The strange skid mark was now explained. One of the rear shocks had broken, apparently putting additional stress on the other shock, causing it to brake. Then the rear fender of the motorcycle dropped onto the rear tire. The rear seat is bolted onto the fender with a ½” bolt. The bolt was rubbing on the rear tire and, in effect, retarding the rear tire’s rotation, just like braking. The bolt was dropping into the cross-grooves in the tire’s tread, causing the mark to get darker, then coming out of the groove and letting the tire rotate a bit easier, then dropping into the next groove.

Finally, the bolt dropped into a groove and locked the rear tire.

After learning of the mechanical defect, it became apparent that this collision occurred because of the mechanical defect…not because of speed, driver error, or anything else. My point, I guess, is that I thought I had a simple wreck that I had figured out minutes after arriving on the scene. I had the typical long skid mark from the back tire, leading toward the outside of a curve. You know, the ones that are left when the motorcycle is flying down the road and the operator brakes with the rear brake and goes off the road. This collision opened my eyes a little wider to the causes of motorcycle crashes. A very, very small percentage of car crashes are caused by mechanical defects. Few mechanical defects in an automobile will cause the driver to completely lose control of the vehicle. However, mechanical defects on a motorcycle, especially tires, wheels, and suspension will almost always lead to a loss of control.

Motorcycle owners frequently customize their motorcycles with aftermarket parts and accessories. Much of that work is done by the motorcycle owner, not a trained motorcycle technician/ mechanic. The owner might not properly install the parts or might use inferior parts in crucial areas of the motorcycle. That can further increase the chances of a mechanical failure.

My point is that I thought I had this collision solved shortly after arriving on the scene. Had I not spoken with the mechanic and learned the history of this motorcycle, I might have “chalked it up” to operator error, speed, etc. After looking a little deeper into the collision, though, it was obvious that this collision occurred because of a mechanical failure on the motorcycle. From now on, I will ask myself more questions, such as, “What’s different about that skid mark?” Or, “Why did that shock break like that?”

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Case Problem: A Multifaceted Approach to Collision Analysis

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Greg Russell, Accident Analysis and Reconstruction Rusty Haight, Collision Safety Institute

ntroduction Reconstructionists universally understand that perhaps the most limiting factor in evaluating a crash is the availability and quantity of concrete, objective physical evidence – “stuff to work from.” Crashes which leave little or no physical evidence to work with often mean the reconstructionist has to adopt broad ranges, general assumptions or “working theories” that may or may not be supported by anything other than the analyst’s opinion. That training and experience notwithstanding, (not to be confused with a reasoned conclusion based on training and experience) broad, general assumptions often leave the reconstructionist in a difficult position articulating the actual rationale for the adopted position and then leave conclusions subject to doubt.

One of the situations analysts encounter where, on the surface, the analysis would seem to be nearly impossible is that involving cars the move from impact to rest leaving little physical evidence to indicate their post impact speeds and, to complicate the analysis, one of the cars gets involved in a second impact on its way to rest.

This case problem offers just such a scenario.

There was a time when the “conventional wisdom” was that in cases involving a secondary contact or where there’s driver activity (i.e.: braking) which brings the car to rest, one “couldn’t do a momentum analysis.” Another bit of that same “conventional wisdom” was that the post impact effective friction for a car in a case suitable for a momentum analysis was one in which the car was slowing to rest post impact at a full 100% braking drag factor.

Thankfully, the ignorant position that mindset exposes has been demonstrated time and again and most reconstructionists have been able to put that aside and focus instead on reasoned principle based on an analysis of the objective information before them. In this case problem, we have a situation where there is little or no indication of the amount of braking/slowing post impact for either involved vehicle and one of the two is involved in a second collision while moving to rest.

Case Background A 1987 VW Cabaret stopped for a red traffic signal is struck from behind by a 2004 Chevy Silverado. On impact, the fuel tank on the VW ruptured and the fuel ignited as it was pushed through the intersection. As the VW traveled across the intersection it veered off to the left before veering back to the right then struck a stopped vehicle in the opposing left turn lane. The Chevy continued to through the intersection before it came to a controlled stop along the center median. The collision scene is depicted in Figure 1. The final rest position of the VW is seen in Figure 2 and the damage to the VW and Chevy truck are seen in Figures 3 and 4 respectively.

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Figure 1

Figure 2

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How are you going to analyze the crash?

As with many collision investigations, the primary question that would be asked was: “how fast was the Chevy traveling when it struck the VW?” That’s the focus of this case problem. You know the VW is stopped so what’s the impact speed of the Chevy truck?

On its face, this may appear to be a question which could easily be answered through the simple application of the conservation of linear momentum: a stopped vehicle was struck in the rear by another; on the surface, one of the most simplistic of momentum problems to solve. However, it quickly became apparent there were numerous issues that would prevent such an approach from yielding reliable results.

The primary issue preventing that otherwise straight forward approach when solving for the impact speed of the Chevy through this use of momentum is the inability to accurately determine its post impact velocity. More specifically, the percentage of braking or slowing for both vehicles experienced after the collision.

After the impact the Chevy traveled approximately 215 feet before stopping. However, there was no physical evidence of post impact braking by the Chevy nor was there any evidence that it was braking prior to or at the time of impact. Nonetheless, since the Chevy did come to a stop, it is apparent there came a point in time where the driver applied the brakes or something related to the collision acted to slow the vehicle. For the purposes of this case problem, you may adopt as fact that the slowing was a function of driver behavior at some point post impact. Attempting to determine when and how hard the driver of the Chevy applied the brakes after the impact would be difficult task, if not all together impossible without more specific, concrete information. For this reason, attempting to determine the Chevy’s post impact speed based upon its post impact travel becomes an unreliable endeavor.

The problems in determining the Chevy’s post impact velocity are mild in comparison to that for the VW. On impact, the rear of the VW was pushed downward causing the rear bumper to gouge more than an inch into the asphalt and the right rear axle broke, allowing the axle to gouge deeply into the road surface (some of that roadway marking is visible in Figure 1). As the VW continued travel across the intersection, it veered right, causing its weight to shift to its left side. As a result of this action the extent of the gouging by the right rear axle slowly diminished until it was no longer marking the roadway. Compounding these problems, during the initial portion of its post impact travel, the rear of the VW was sliding on the fuel from the ruptured fuel tank. Adding further to the complications is the fact that the VW drove head-on into another vehicle, prematurely halting its post impact travel.

In order to determine the post impact velocity of the VW based on its post impact path of travel, its impact velocity from the collision with the stopped vehicle across the intersection would have to be determined first. This would have to be accomplished through a separate momentum analysis. After this analysis was completed the post impact velocity loss from the first impact to the second would have to be determined and combined with the second impact velocity to arrive at the VW’s post impact velocity from the initial collision.

The problem with this analysis would be in quantifying the constantly changing post impact braking percentage of the VW as it traveled across the intersection from being struck by the Chevy truck. Therefore, as with the Chevy, attempting to determine the VW’s post impact velocity based upon it post impact travel becomes unreliable at best.

In this case, we also have available a Bosch Crash Data Retrieval (CDR) System report for data stored in the airbag control module of the Chevy truck. Figure 5 is selected pages from the CDR report reflecting the data recovered from the Chevy. It is, of course, not the complete report but, for the purposes of this case problem, includes the “Data Limita44 Collision

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tions,” deployment system status data and delta-V data for the Chevy as a function of the subject collision.

So, there you have it: a truck rearends a stopped car. The truck comes to a driver controlled point of rest some distance from the impact. The passenger car moves from impact some distance before striking a third vehicle before coming to rest. You have some crash data to work with but lacking some other independent finding of impact speed, how are you going to adopt that data as reliable in this instance? How are you going to find the truck’s impact/ closing velocity?

Figure 3 Figure 4

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System Status At Deployment SIR Warning Lamp Status Driver's Belt Switch Circuit Status Passenger's Belt Switch Circuit Status Passenger Seat Position Switch Circuit Status Ignition Cycles At Deployment Ignition Cycles At Investigation Maximum SDM Recorded Velocity Change (MPH) Algorithm Enable to Maximum SDM Recorded Velocity Change (msec) Driver 1st Stage Time From Algorithm Enable to Deployment Command Criteria Met (msec) Driver 2nd Stage Time From Algorithm Enable to Deployment Command Criteria Met (msec) Passenger 1st Stage Time From Algorithm Enable to Deployment Command Criteria Met (msec) Passenger 2nd Stage Time From Algorithm Enable to Deployment Command Criteria Met (msec) Time Between Non-Deployment And Deployment Events (sec) Frontal Deployment Level Event Counter Event Recording Complete Multiple Events Associated With This Record One Or More Associated Events Not Recorded

Seconds Before AE -5 -4 -3 -2 -1

Vehicle Speed (MPH) 57 57 57 56 55

Engine Speed (RPM) 1536 1536 1536 1472 1472

OFF BUCKLED UNBUCKLED Rearward 2813 2815 -22.10 120 10 N/A Suppressed N/A N/A 1 Yes No No

Seconds Before AE -8 -7 -6 -5 -4 -3 -2 -1

Percent Throttle 0 0 0 0 0

Brake Switch Circuit Status OFF OFF OFF OFF OFF OFF OFF OFF SDM Recorded Velocity Change (MPH)

1GCEC14T54Zxxxxxx Deployment Data 0.00

-10.00

-20.00

-30.00

-40.00

-50.00

-60.00 10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

Time (milliseconds)

Time (milliseconds)

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

Recorded Velocity Change (MPH)

-1.55

-4.65

-7.13

-9.30

-10.85

-13.02

-15.19

-16.74

-18.60

-20.15

-21.70

N/A

N/A

N/A

N/A

1GCEC14T54Zxxxxxx

Page n of 7

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Centrifu g a l Fo C e n t r i f u g a r l c e e o

F l c a r g I u s f i r o t I t n e R F C e Is It iRfuegaaaall??ll F ? r l g a t e R t I n u s I f e

i C ntr Reaall?? t e I C Is It Re s I

By Peter H. Rast

f course centrifugal force is real! We feel it every day! We’ve probably all spun an object on the end of a string and felt the outward pull on our arms and the force we have to exert on the string with our hands. We feel this common force when we drive around corners and slide outward in our seats. It’s one reason motorcycles and bicycles lean in turns. It may also be the force that causes weight shift as vehicles round turns. And, student pilots learn that it is equal and opposite to centripetal force in an airplane’s turn. So why even ask the question when the answer appears to be so obvious? If one knows the answer, one probably understands Newton’s Laws of Motion. And, one may also know about d’Alembert’s Principle.

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Accident reconstruction practitioners are well aware of Sir Isaac Newton and his Laws of Motion. Reconstructionists are probably less familiar, if at all, with Jean le Rond d’Alembert. This is probably because d’Alembert is better known for his contributions to science and mathematics involving fluid dynamics and refraction.

He is also known for the d’Alembert system of gambling and betting that is based on the (incorrect) gambler’s fallacy. But, most probably, we aren’t familiar with d’Alembert because his modification and expression of Newton’s Laws is based on a fiction that may be useful in limited circumstances, but doesn’t add anything to our understanding of motion. ir Isaac Newton and his Laws of Motion Sir Isaac lived from 1642 to 1727. During this time he formulated the law of gravitational attraction and the Laws of Motion that we know so well. To understand the origin of the concept of centrifugal force and what d’Alembert did we should first review Newton’s Laws.

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Newton’s First Law of Motion is actually a special case of the Second Law (Rast, 2008). The First Law tells us that an object will maintain uniform motion unless acted upon by an outside force.

This is often called the law of inertia. It tells us that if no force is exerted on an object or, more to the point, if the forces exerted on an object sum to zero, the object will not accelerate, decelerate, or change direction, i.e., turn. (hint: remember this. There will be a quiz later!)

Newton’s Second Law has several parts. Expressed as we commonly see it, this is the Second Law: F = ma Newton presented the Second Law in this form:

a=F/m

and explained it this way: an object’s acceleration, a, change in velocity (speed and/or direction), is proportional to the net force, F, exerted on it and inversely proportional to the object’s mass, m.

Note that both formulations speak of change in an object’s motion. Newton’s Laws are expressions of dynamics, i.e., objects in motion.

Consider what would happen if the forces, F, exerted on an object sum to zero. If F is zero, then a must also be zero. In other words, as the Second Law states, no net force = no acceleration. Since direction change and speed change are both accelerations, no net force = no change in direction or speed. Newton’s Second Law tells us that an object would continue in uniform motion as if no force is exerted: the First Law. Newton’s Third Law states that two objects in contact exert equal and opposite forces on each other. This is one source of the confusion about centrifugal force (although Newton probably didn’t use

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the term). Consider the example of a car turning or going around a curve. In such turns, we have all felt the sensation of slipping to the outside of the curve. It has been suggested to us that this sensation is due to centrifugal force acting opposite to the turning force and it is further said that, because of Newton’s Third Law, this force is equal to the turning force. But, what object is in contact with the car or anything in it that would give rise to a force equal and opposite to the turning force? The key to understanding Newton’s Third Law and its application is to realize that contact between objects is required! No contact - no equal and opposite forces!

ean le Rond d’Alembert and His Principle Jean le Rond d’Alembert lived from 1717 to 1783. He was ten years old when Newton died and surely knew of Newton’s Laws and the calculus. However, even many years after Newton’s principles and the calculus were known, solving dynamics problems remained difficult and usually relied on the more commonly understood methods of static analysis. D’Alembert’s Principle is essentially one of converting a dynamics problem to a statics problem so that it could be solved with the more commonly understood methods of statics (Meriam, 1966).

As another example, student pilots are taught that centrifugal force must be overcome for an airplane to turn. Consider the following statements from the FAA’s 1980 Pilot’s Handbook of Aeronautical Knowledge (Department of Transportation,1980):

Newton’s Laws related the motion of objects to a fixed reference system within which the objects moved such that their accelerations with respect to the fixed system and to other objects in the system could be observed and described. Such a construction is fundamental to Newton’s Second Law: F = ma

“During the turn entry..........the horizontal component (of lift) must overcome centrifugal force.” “...as the turn develops, centrifugal force will act opposite to the horizontal component of lift...... So long as these opposing forces are equal to each other in magnitude, the airplane will maintain a constant rate of turn.”

Now consider the following statements from the FAA’s 2003 Pilot’s Handbook of Aeronautical Knowledge (Department of Transportation, 2003): “There can be no unbalanced forces in steady, straight flight (Newton’s Third Law).”

“The force of lift during a turn is separated into two components at right angles to each other. One component, which acts vertically and opposite to the weight (gravity), is called the “vertical component of lift.” The other, which acts horizontally toward the center of the turn, is called the “horizontal component of lift,” or centripetal force. The horizontal component of lift is the force that pulls the airplane from a straight flightpath to make it turn. Centrifugal force is the “equal and opposite reaction” of the airplane to the change in direction and acts equal and opposite to the horizontal component of lift.”

“In a correctly executed turn, the horizontal component of lift must be exactly equal and opposite to the centrifugal force.”

A final example from a well-know accident reconstruction text defines centrifugal force thusly: Centrifugal force, the force of a body in motion which tends to keep it continuing in the same direction rather than following a curved path (Fricke, 1990).

But, if centrifugal force actually exists, how does a car or airplane actually manage to turn? If centrifugal force, the outward directed force, is equal and opposite to centripetal, the center seeking, force, the net turning force when the two forces are summed must be zero. Newton has said that if the net force on an object is zero, the object will not accelerate, i.e., neither its speed nor direction will change. The bottom line is this: if centrifugal force exists and is equal and opposite to centripetal force, cars and airplanes, or anything else in motion, would never turn. The inescapable conclusion derived from Newton’s First and Second Laws is that centrifugal force does not exist. To suggest that it does exist based on Newton’s Third Law is a misunderstanding of Newton’s Third Law. For pilots, it should be clear why the statement: “There can be no unbalanced forces in steady, straight flight (Newton’s Third Law).” is incorrect. The statement itself is correct; the attribution to Newton’s Third Law is not.

J

However, d’Alembert observed that

is also true. d’Alembert’s formulation is a static formulation that is the same as saying the reference system of interest is attached to the object in question and moves with the object. The result is an object that has no motion with reference to the d’Alembert reference system, is in equilibrium, and can be treated with the methods of statics.

While the force, F, is real, the equal and opposite force, - ma, is not (Sears, 1950). Only in the d’Alembert Principle is this force found. It is a fictitious force or construct developed by d’Alembert to make it possible to treat a dynamics problem as a statics problem; d’Alembert knew it was fictitious. Centrifugal force, as we’ve come to call d’Alembert’s fictitious force, is also known as the inertial force and the artificial state of equilibrium created by employing this fictitious force is known as dynamic equilibrium (Meriam, 1966). Note that the definition of Centrifugal force quoted above seems to be what might be expected to define an inertial force consistent with Newton’s First Law. here Does This Leave Us? Today, d’Alembert’s Principle is primarily one of historical interest, is seldom taught, and adds little to the analysis of dynamics problems (op cit). However, the concept of centrifugal force, the inertial force, continues to dog students, drivers, prospective pilots and experienced pilots, among others. But, even d’Alembert realized that the force he postulated, the force that was equal and opposite to centripetal force, was fictitious. So, while we now know that centrifugal force is a fiction, the question remains: what is this mysterious phenomena that seems to drag us outward in a turn? That’s the quiz question. The answer is in Newton’s First Law. Or, is it in Newton’s Second Law? Hint: it’s not the Third Law.

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References Department of Transportation. (1980). Pilot’s handbook of aeronautical knowledge (EA-AC 61-23B). Oklahoma City, OK: Federal Aviation Administration.

Department of Transportation. (2003). Pilot’s handbook of aeronautical knowledge (FAA-H-8083-25). Oklahoma City, OK: Federal Aviation Administration.

Fricke, L. B. (1990). Traffic accident reconstruction. Evanston, IL: Northwestern Traffic Institute.

Meriam, J. J. (1966). Dynamics. New York: John Wiley & Sons.

Rast, P. (2008). Energy and momentum in accident reconstruction: a code 3 case study. Salem, OR: Kinetic Energy Press.

Sears, F. W. (1950). Mechanics, heat, and sound. Reading, MA: Addison-Wesley Publishing Company.

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Basic Integral Calculus for Crash Reconstruction Sean Haight Texas Christian University

College of Engineering

Collision Safety Institute

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There is an obvious link between Newtonian physics and crash reconstruction; however, the link to mathematics is not as intuitive.

Among other things, basic math is commonly used when adding vectors, evaluating crush, and converting between imperial and metric units. This article examines how fundamental calculus techniques can help define reconstruction variables, specifically delta-V (velocity change). As a refresher for some and first exposure in the application for others, this is a look at calculus and the definite integral in the context of crash analysis.

Starting with the last question first, say you have a set of acceleration data from a crash test or imagine you download crash data from a car and it offers a crash pulse but not the delta-V. What are you going to do with the acceleration data? Simply put, you can calculate the “area under the curve” and find the delta-V. That delta-V is then used, of course, with other calculations to find closing velocity and, in some cases, impact velocity. Before any of that; however, we have to work on getting from acceleration to delta-V and, to get there, we’ll cover some basics first.

But why? As crash data becomes more common and more auto manufacturers (OEs) make their stored data available, we will most assuredly see that the data storage schemes will continue to vary between manufacturers and parts suppliers. As part of the differences between OEs, we already see how some record delta-V differently. Some OEs store delta-V while others store acceleration. For example, General Motors vehicles typically store delta-V in 10ms “blocks” for varying crash pulse periods and Ford vehicles store acceleration which is then used by the Bosch Crash Data Retrieval (CDR) software to calculate delta-V. But, how is that calculation done? What’s necessary to take a set of acceleration data points and convert that to some quantity of delta-V? When would I want to even do that sort of analysis?

We can start by reviewing the fundamentals of an acceleration pulse. Figure 1 is an example of a crash pulse captured during a frontal crash test. The horizontal axis is time in seconds, and the vertical axis is acceleration in g’s. There are several data points that are important in this plot. For example, the measured max acceleration is around 14.5g, and the crash duration is about 96 milliseconds. However, cumulative delta-V can’t be found just by looking at the graph and considering a maximum value so this is where calculus comes into play and where we contemplate the “definite integral.” For our purposes, the integral will be a number that represents the area enclosed by a time based curve, the time axis, and two time points.

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Figure 1: Example frontal crash test acceleration trace as a function on time Integral calculus is a branch of calculus that focuses on calculating the area and volume of irregular shapes. A crash pulse is definitely going to be an “irregular shape.” The “definite integral” is a value calculated between two specified limits. This is clearly not something “new,” the principles of the integral were first used by Isaac Newton and Gottfried Leibniz in the late seventeenth century. In the most basic terms, the integral is the “area under the curve.” This means, when you have an acceleration plot as seen in Figure 1, the area of the acceleration trace is equal to the cumulative delta-V.

an acceleration pulse (curve) like the one found in Figure 2 is measured and you are given the task of calculating a delta-V from the data. You may notice that the simple pulse depicted in Figure 2 is positive - the acceleration is positive as graphed. Offered here as a reminder, SAE J1733 and SAE J211 establish the vehicle fixed axial system and, in that context, we might recognize that the concepts offered here apply whether the car is struck front-to-rear (as seen in Figure 1) or “rear ended” (struck rear-to-front as might be depicted in Figure 2).

Finding the integral might be compared to a simple math operation like addition, subtraction or division. In its simplest form, it is defined by the equation:

For this example, let’s assume that you don’t have any software or a graphing calculator; just a pencil, paper, and a basic scientific calculator. Since we know that the area enclosed by the triangular acceleration pulse is equivalent to the cumulative delta-V, we must first analyze the boundary conditions. For this example, everything below one second, and greater than 3 seconds can be ignored because the values beyond that boundary are zero.

where a(t) is the acceleration signal over time. The “dt” term represents that the area is being calculated is with respect to time.

Let’s first take a look at a very simple example: a crash occurs and

For the example, then, our boundary conditions are 1s ≤ t ≥ 3s. In mathematical notation, this would look like

The next step is to find the area of the curve from the time 1 second to time 3 seconds. Since this signal is simply a triangle, we may recall middle school geometry and write the equation for the area of a triangle as: area= 1/2*base*height.

Here, since the base is two seconds and the height is 4 g’s, we can say that the area of the triangle is 4. Now, here is where units come into play, acceleration in g’s is a function of the fundamental constant of gravity. For imperial units, it is 32.2 feet per second squared and for metric units, it is 9.8 meters per second squared. Looking only at the math for the moment, when we multiply the gravity constant by the area, 4 times 32.2 equals 128.8. This means that our calculated value is 128.8 and then, for the units we’re using, the delta-V would be represented as +128.8 feet per second.

Figure 2: Simple triangular acceleration pulse

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Admittedly, this is an unrealistic data set as it relates to crash reconstruction because finding a crash that actually lasts crashes lasts 2 seconds is, at best, uncommon, but it illustrates the concept in simple “in your head” math. The next example will show how an actual crash pulse may be used to calculate delta-V.

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Figure 3: Enclosed area divided into 3 millisecond squares in the midpoint configuration The acceleration trace in Figure 1 will be used in this example to calculate delta-V. As in the previous example, the time boundary conditions must be established first. Considering the crash pulse itself, we will, of course, exclude pre-crash braking. Looking at the graph offered as Figure 3, we can say that the total crash pulse lasts about 96ms (.096s). In math notation, this would look like:

The next step is to find the area that is enclosed by that time boundary. In this example it is more difficult than simply finding the area of a uniform triangle. For non-uniform shapes, we will use a technique called the Riemann Sum method. This technique is used to find the area of waveforms that are hard to define by more basic geometric shapes and simple mathematics. It consists of dividing the wave shape into many partitions of defined, finite time. For example, we can draw more regular shapes based on 3 milliseconds widths on the graph’s time axis and repeat that process many times.

As demonstrated in Figure 3, the acceleration pulse can be separated into 32 distinct rectangles.

The alignment of the rectangles to a starting point is another decision that must be made when using the Riemann Sum method.

Where should the left hand of the first rectangle touch the edge of the acceleration curve, should it be more to the right, or should it touch a midpoint of the rectangle? In fact, all these methods can be used. Especially in the case of a crash pulse where the slope of the acceleration is nearly equal on both sides and therefore over or under estimation of the delta-V is less affected by that choice so long as the application is consistent across the pulse itself. For most cases involving the kinds of data we see from a crash, it’s probably best to use an approach where the first rectangle is intersected at the midpoint of its width. For comparison, the difference between the midpoint and the left point approximations can be seen in Figures 3 and 4.

Another design consideration relates to the amount of time each rectangle covers. The larger the rectangle, the higher the risk that the delta-V will be over or under estimated. In reality, the time range that is chosen is not as big of a factor as one might think. For this example, we can consider two different time ranges (three and six milliseconds). Figures 4 and 5 show the graphical difference between the two ranges.

Figure 4: Enclosed area divided into 3 millisecond squares in the left hand point configuration www.collisionpublishing.com

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Figure 5: Enclosed area divided into 6 millisecond squares in the midpoint configuration Once the sizes of the rectangles are chosen, the area of them can be calculated. From geometry, the area of a rectangle is: area=1/2*base*height. the base of the rectangle is the time range that we have previously chosen (i.e.: 3 or 6 milliseconds). The height of the rectangle is the acceleration value at a specific time instant. For the midpoint approximation, the height value of the rectangle is the value of the acceleration at one half of the time interval for the specific rectangle.

The area of each rectangle can be calculated separately and each of them can be added together to get the total area under the acceleration curve. This simply means that the area of each individual rectangle can be added together to calculate a total area. Since the units are in g’s, they must be converted to feet per second squared or meters per second squared. When the total area is calculated, you have found the delta-V for the crash.

For this example, the delta-V has been solved using various approaches to show that they can all be applied and the variation between each. Table 1 shows these different methods and their calculated delta-V. It also shows the actual delta-V as calculated by the software for the accelerometer’s used in the crash test.

As noted in Table 1, a trapezoidal method as seen in Figure 6 can also be implemented to find the area under the curve. This method used the same procedure as the others; however, it uses the area of a trapezoid instead of a rectangle. Another commonly used geometry for calculating the area is a single triangle (similar to the approach discussed earlier and as seen related to Figure 2). This can be used as a “quick and dirty” approach since it usually has higher error rate (see Table 1) but the results are easily found in a minimal amount of time. For this crash pulse, the triangular method is shown graphically as Figure 7.

Looking at the data, whether using a 3 millisecond or 6 millisecond rectangle width, the difference between the calculated values and the actual value is less than 3 percent. When one uses the rectangle widths on the order of 12 milliseconds, there is a risk of higher error, as in the case of the 12 millisecond left hand calculated point data set. Since each set of acceleration plots are different, each type of geometry will provide different results and different variation from the true value. More specifically, for this crash, the method that most closely approximated the calculated value found using the accelerometer’s software was the 6 millisecond midpoint method. However, for another crash pulse, the 6 millisecond left hand

Figure 6: Enclosed area divided into 6 millisecond squares in the trapezoidal configuration 60 Collision

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Actual delta-V = -17.18 mph Geometry 3ms rectangle left hand point 3ms rectangle midpoint 6ms rectangle midpoint 6ms trapezoid 12ms rectangle left hand point Single triangle

delta-V (mph)

Difference* (mph)

% difference*

-17.456 -17.562 -17.448 -17.533 -17.920 -15.229

-0.276 -0.382 -0.268 -0.353 -0.740 -1.950

1.61% 2.22% 1.56% 2.06% 4.31% 11.35%

Table 1: Calculated delta-V using six different geometry types. (*difference shown relative to accelerometer’s software calculated delta-V) point might actually be a closer approximation to the true value.

The author’s recommendation is to use 6 millisecond increments or less when calculating delta-V for smaller periods such as those found in crashes.

For the measurement of the heights and widths of the rectangles, many methods can be used. The easiest is to actually print the graph and do it manually with a ruler and pencil. Make sure that your units are being kept in mind when measuring. Other methods including a more advanced method utilizing an imaging program like Adobe Photoshop to measure the amount of pixels along the side of the rectangle and relate that to a pixel per second or pixel per g ratio. When using this method, make sure not to stretch the image so that the relationship between time and acceleration is held constant.

is an easy way to do calculations and keep all of the individual rectangle areas in proper order. The options offered here demonstrate that a calculation for delta-V from raw data might be approached in different ways and, so long as the end user keeps certain constraints in mind, the end result comparison of the various methods should be approximately the same (keeping in mind the basic triangle method’s obvious limitations).

As reconstructionists deal more and more often with acceleration data, whether from a crash as recovered using the CDR System or from a crash test, finding delta-V is the first step in using that data to ultimately find other values useful in the crash analysis including closing or impact velocity.

Another potential approach would be to sue a CAD program and, after importing the data graph and then outlining the actual area in that program, one may use the CAD program’s offer option for actually calculating that area as then defined. Similarly, altering the aspect ratio of the figure when imported will affect the result.

For more closely calculating the individual values of the individual partitions, I recommend using a spreadsheet program like Excel. It

Figure 7: Enclosed area represented by a single triangle www.collisionpublishing.com

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Forensic Photography and Crash Reconstruction Part 1:

“The Professional Amateur Expert Witness Photographer”

By Robert Wyman f you’ve ever read these words in a depo transcript or heard these words during live testimony, you should have next been fighting to suppress a smile. When an opposing expert cannot produce tangible evidence in support of their conclusions, opinions and testimony, words are all that remain.

I

You’ve been handed an opportunity on a silver platter.

Armed with your own properly-executed photographs, you can exploit the opposition’s failure and convince the court that your conclusions and opinions are substantiated by evidence. That evidence is now tangible, portable, colorful, clear, concise and compelling.

• The evidence can be easily observed and accurately analyzed.

• It can be measured and compared.

• It illustrates the complex mathematical calculations which serve as the basis for your conclusions.

• It dispels the improbable “what if ” scenarios which are inevitably thrown your way.

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The photographic images are not evidence. They only illustrate the evidence … in a “fair and accurate” manner. Tire marks, fluid spills, debris patterns and gouges cannot be reproduced on the courtroom floor. Sheetmetal crush, automotive components, wet roads and obstructed traffic signs cannot be brought into the jury room.

Biological evidence, injury mechanisms and the injuries themselves cannot be placed on a table for inspection.

But, your photographs can do all of these things. ntentions And Results My intent here is to simply discuss the importance of the photographic task. I am not here to promote my photographs as being any better than yours, and I have no interest in boring you with issues of focal lengths, f-stops, histograms and camera settings.

I

In fact, your best strategy is to learn enough to validate an image as being “fair and accurate” without delving into such technical camera details, since anything less than a complete understanding will certainly be problematic under cross-examination. So, don’t worry about the fact that you’ve never turned your camera’s function dial away from that green “AUTO” setting. After all, AUTO is meant

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for car pictures, right? Says so whenever I look at the camera!

Let’s get back to the example above. Have you ever said those same words, “This picture really doesn’t show it well, but…”

If so, please recognize your failure. You attempted to record an image of something deemed significant to your case, yet that image cannot be relied upon to convey the proper message to your audience. Is your failure a detriment to your case? Will it adversely affect your other testimony points? Will the ineffectiveness of this single image result in a verdict supporting your opposition?

Why take a chance with such outcomes when the photographic task is so simple?

Actually, it’s this view of simplicity which results in failure. If the image isn’t effective, the camera is usually blamed: • “Yeah, it was already dark outside by the time I took these pictures, and for some reason you can only see these license plates and reflectors but not the crush. It was a dark blue car, though, so maybe that was why the pictures didn’t come out right.”

• The next excuse is similar, citing the camera plus the lighting: “It looks dark in the picture, but it was really light outside. I mean bright, sunny, not a cloud in the sky.

I was trying to show the seatbelt inside the car. I don’t know why the camera did this and made it so dark.”

• The final popular excuse is one of size and scale: “Yes, the surface scratches, abrasions and gouges are in this area of the picture. You can barely see the gouge right there. I measured that angle and used it in my calculations. It’s hard to see here, but it goes off in a northwesterly direction. It’s a lot clearer in my CAD diagram.”

Well. First of all, the camera is not to blame. In each example, the camera did exactly what it was supposed to do and programmed to do. Like with many things in life, failures were due to “operator error” and not a mechanical, electrical or logic fault of the camera. The operator had a particular image in mind, with the intention of recording that image as seen in the mind’s eye, but failed to utilize the camera in a way to produce the intended result. quipment Proficiency Switch gears with me for a moment. In terms of your on-duty weapon and/or other firearms, there is little that you don’t know about them in terms of performance, tactics, and even in-situ expectations. Same with your vehicle. And your radio. And your total station, accelerometer, CAD software, calculation software, animation software, cellphone and cool new LED flashlight*.

E

* Please notice that I specifically didn’t mention VCRs, programmable microwave ovens, clothing irons and some old computer networks, each of which truly do require an advanced linguistics degree to decipher the owner’s manual.

Since you’ve taken the time to recognize that each piece of equipment requires unique training for establishing and maintaining proficiency, why do you expect perfect performance from a camera, in every situation, after only turning it on and pressing the shutter button?

Your camera is a tool just like everything else in your recon kit. Proficiency comes with knowledge, practice and application. If your nighttime scene photos never turn out to be anything more than black blobs with pinpoints of light reflecting from license plates and traffic signs, then realize the solution rests with the photographer more so than the camera.

Of course, cameras and their resulting images, especially those used in a forensic environment, are subject to similar considerations (and challenges) faced by your other equipment. Images must be “explainable” and the aforementioned fair and accurate.

• Fairness is generally subjective. If anyone else at the same scene could have taken the same image with the same or similar camera, without any specialized equipment or imageprocessing tools (hardware or software), then the image is a fair representation of what was seen with the unaided eye. It is not prejudiced in any way, and there is no intent to perpetrate a fraud by misleading the viewer.

• Accuracy is another story, and often difficult to quantify at an amateur level. An accurate image is generally one in which measureable elements of the image are a reasonable match to measureable elements of the scene. Items such as color, contrast and brightness can be quantified numerically or anecdotally. Items involvwww.collisionpublishing.com

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ing perspective distortion, lens distortion, or other optical attribute can be similarly documented.

In practice, the subjective elements of fairness encompass and surpass the objective elements of accuracy: if the photo indeed looks like the scene, then it’s usually deemed an accurate representation without the need of further measurements and validation. quipment Limitations As you already know, however, the camera does not see what the eye can see. A Google™ search of something like “megapixels comparable to human vision” will unearth studies, articles, opinions and arguments taking a full day or more to read and understand. Numbers upward of 500 Megapixels are currently offered as an equivalent value to human binocular visual acuity, but this figure carries various explanations and caveats regarding the eye’s rotational movement, field of vision, focusing mechanism, anatomical features as related to target acquisition, plus attributes such as the brain’s image-processing power, memory, pattern-analysis and cognitive abilities.

E

So as not to overstep my bounds, I’ll defer to the well-respected Human Factors researchers and authors among the readership to provide a more detailed camera vs. eye analysis. levating The Photographic Task We are now almost full-circle from the title and opening sentence. If your pictures do not properly illustrate the subject at hand, you have an obligation to correct the problem. There should never be a transcript where you admit that one of your photos is erroneous or ineffective, because you have many resources available for learning and practicing the type of photography which your job demands.

E

Yes, you can learn…

• To take nighttime photos which properly illustrate dark-painted vehicles on dark asphalt with black skid marks.

• Why a vehicle’s interior looks so dark even on a sunny day, and how to remedy the situation.

• To get your camera down to street level and make that gouge look like the Grand Canyon… or not.

• To capture a VIN through the windshield without seeing your own reflection in the glass, no matter how good looking you are.

• To document a lamp filament with enough magnification to easily see oxide deposits, or even see that a single strand of hair embedded in a windshield was artificially colored or bleached.

Fingerprints, shoe prints, seatbelt loading, blood spatter, debris patterns, gouges, crush deformation, component failures, roadway defects, traffic control device issues, sight-distance issues and almost everything else can be documented photographically, even if other forms of measurement and documentation are employed. All it takes is a little knowledge, some creativity, a decent collection of equipment, the time to do the job correctly… and a mindset that accurate photography tasks are just as important as accurate measuring tasks. These are recon-staff skills just as much as they may be CSI-staff skills.

world. Those “thousand words,” i.e., “a picture is worth a thousand words,” can easily represent about ten minutes of testimony. So, if a compelling image is displayed and the jurors start to shake their heads affirmatively, without you having said a word so far, I believe you can claim the thousand-word trophy of achievement.

Conversely, displaying an enlargement of a poorly-executed photograph will only result in the jurors’ confusion and distraction as they attempt to figure out what’s in the picture… and what you’re now talking about during testimony… and why you’re harping on this gouge-thing that they can’t even see… and why the angle or whatever is so important to that calculation you mentioned… what was it called… momentary, momentum, moment sum? “Oh, I don’t know what that expert is talking about!” re You An Amateur Or A Professional?

This leads me to a final point: Are you a professional photographer?

A

In recent classes and seminars which I’ve conducted, the answer is usually “no.” Few raise their hands when asked if anyone in the room considers themselves to be a pro photographer. No one ever raises their hands when asked, of the pro photographers, “…how many are forensic photographers?”

Attendees by and large consider themselves hobbyists or amateur photographers, but certainly not professionals and especially not forensic photo experts.

Don’t sell yourself short. Whether you like it or not, you are a professional photographer and a forensic photographer. You may not consider yourself to be particularly skillful, but you are nevertheless a pro. Why? Because you’re receiving compensation for the photographic task, and that task is being used in an evidentiary environment. It’s either required of you based on policy and job specs, or a necessary service you’ve chosen to provide. Either way, it becomes part of your expertise.

As for skill, don’t sweat the minor issues. Is a Computer Forensics Detective more skillful than a Domestic Violence Detective? Is a Traffic Homicide Detective better than a Burglary Detective?

(Note: Don’t answer that question out loud) You may not possess the creativity of a wedding photographer, the studio of a fashion photographer, or the technical prowess of an architectural photographer, but you still do things that an average person cannot do: find and record critical evidence for the LEO and legal communities.

• On the stand, when asked if you’re a professional, always answer “yes.” Because you are.

• When then asked to explain f-stops and focal lengths and histograms, because you’re such a professional photographer, please sit up straight, smile broadly, and say with all confidence that your assignments simply don’t require such a detailed level of knowledge in order to capture evidence in an expert manner. Because that’s the truth.

Enjoy your new title, “Professional Amateur Expert Witness Photographer,” and order some new business cards to celebrate!

he Payoff Now that you’ve decided to make better pictures, what’s the payoff? In terms of testimony, what’s the value of your photo to the case?

T

You know the answer, and it certainly rings true in the recon 64 Collision

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9

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Display Average Table 2/18/2008 Date & Time 9/24/2002 09:18 AM 9/24/2002 09:19 AM 9/24/2002 09:21 AM 9/24/2002 09:22 AM 9/24/2002 09:23 AM 9/24/2002 09:25 AM 9/24/2002 09:27 AM Average of Runs

Run No. 4 5 6 7 8 9 10

Time Sec 1.95 1.95 1.95 3.46 3.41 3.35 4.13

Acceleration Average G -0.891 -0.887 -0.882 -0.897 -0.897 -0.895 -0.892

Speed MPH 38.10 37.95 37.74 68.07 67.06 65.74 80.80

Distance Feet 56.6 55.8 56.2 178.5 172.4 168.4 253.4

2.89

-0.891

56.50

134.5

f 0.80

17

18

19

Range Table Speed Result 57.88

0.81 0.81 0.83

58.24 58.60 58.96

0.84 0.85 0.86 0.87 0.88

59.31 59.66 60.01 60.36 60.71

Phone 763-428-1381 USA or Canada 800-533-5547 fax 763-428-1381 info@vericomcomputers.com www.vericomcomputers.com 14320James Road - Suite 200 - Rogers, MN 55374

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School Bus Rollovers and Passenger Compartment Ejection: Solving the Puzzle Using Videoanalytics Dr. Ray Turner School Bus Accident Reconstruction, LLC.

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ifferent School Bus Body Types---All with Common Rollover Vulnerabilitues Rollover vulnerability of school bus types, makes, models and years have not been sufficiently addressed in the accident reconstruction community. Our concern for rollover and roof crush factors in automobiles, LTVs and SUVs has not been matched with an equal concern for our children and the rollover vulnerabilities of school buses. 23,500,000 students are daily passengers on school buses. Few parents realize that their child is in a vehicle with a higher center of gravity than many other vehicles on the road. School bus roof crush protection and roll cage construction may lead the public to a false conclusion that school bus passengers have an inherently lower risk of injuries while being unbelted during bus collisions. Not so with school bus rollovers. The school bus industry points to relatively minor injuries to student passengers in all types of bus collisions1.

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School bus advocates ignore the fact that rollovers produce serious if not fatal injuries2. Students in relatively low-speed rollover collisions were previously thought to receive only minor injuries. However, three times more head injuries than those previously reported have been found by competent medical researchers3. Serious passenger school bus passenger injuries have been and continue to be misrepresented and underreported4. The most common serious injuries of student passengers are head injuries in rollover collisions5.

Accident reconstructionists had no proof of actual collision events and were obliged to predict collision events. Rollover injuries and their causes and effects can now be observed and studied with onboard school bus videotape data. “Seeing is believing” is now available to collision investigators, accident reconstructionists, attorneys, juries and the general public to see and better understand the puzzle of collision events using videoanalytics6.

Using VIdeoanalytics to Determine Passenger Ejections within the Bus. Careful analysis of onboard bus videotape tells a much different story when forensic videoanalytics technology is used to resolve poor tape quality down to the pixel-by-pixel level. Digital recording of school bus collisions provides a much higher resolution than older videotape (VHS) technology previously used on the bus. Buses are now equipped with front, rear and overhead digital views providing multiplexed data for a view—by view or camera-by-camera videoanalysis7. School bus occupants without seatbelt restraints experience significant displacement forces that can now be analyzed row-by-row and student-by-student throughout all phases of the onboard collision event8. Close-range photogrammetry and reverse projection techniques together enable investigators to overlay a three-dimensional bus interior with occupant kinematics showing passenger seating, body positions before, during, and after the collision. Every school bus in America with a videocamera can now repurposed to provide collision investigation data using forensic videoanalytic technology. “CSI” is now on the school bus.

School Bus Rollover Phases and Occupant Kinematics. Moffat described a three-phase occupant kinematic of rollovers of passenger cars and light trucks including the tripping phase, a roll phase and ground contact phase9. Videoanalysis of rollovers suggests a seven phase system to describe occupant kinematics during school bus rollovers (FLOAT, THROW, COLLIDE, OCCUPANT SLIDE, BUS POINT OF REST, OCCUPANT POINT OF REST and ending with UNTANGLE). Each of these seven stages can be observed in videotaped rollovers. One of the most widely viewed videotape quarter-turn rollovers was presented in October, 2007 and has been repeated in many video viewing web pages10.

Moffat’s study included belted and unbelted passenger in autos and LTV rollovers where unbelted occupants tended to tilt in the direction of the vehicle movement. Unless they are contained during rollovers within the compartmentalization envelope (CE) that is formed by high-backed bus seats, students will be ejected out of their seats to begin their FLOAT PHASE. The further away from the bus center of gravity the greater the degree of those student’s

airborne displacements during the FLOAT PHASE. During Moffat’s Roll Phase unbelted passengers moved upward and outward away from the auto or LTV center of gravity. Bus rollover victims already FLOATING can be observed to be ejected within the bus in the THROW PHASE. Moffat’s last ground contact phase does not account for school bus passengers who continue to move at the original velocity of the bus until they contact bus interior surfaces in the COLLIDE PHASE. But the rollover collision is not yet over for them. During the COLLIDE PHASE digital video recordings reveal passengers colliding with other passengers as they are thrown about within the bus. Other bus passengers in their own COLLIDE PHASE trajectories strike one or more bus interior surfaces and not other passengers.

Passengers having collided with bus surfaces alone, or with other passengers, remain in their positions for relatively long intervals in the OCCUPANT SLIDE PHASE continuing on after the bus has reached it’s final point of rest (VEHICLE POINT OF REST). Bus OCCUPANT POINT OF REST extends past the vehicle point of rest and includes student injuries resulting from their untangling after contact with other passengers.

Rollover Injuries can and do often occur in post-accident student self-evacuation or extraction, rescue and evacuation by others. For example, in Cotulla, TX a lift bus struck from the rear by an 18wheeler at a high rate of speed caused the bus to flip end-over-end twice, and longitudinally rollover repeatedly until it came to rest upside down (See photo). Special needs passengers, all of whom were belted in, experienced additional injuries after the vehicle arrived its the point of rest upside down. Occupants within fell to the ceiling when releasing themselves from their seatbelts or were further injured in falls after rescuers released them from their own securements11. The last rollover collision phase is the UNTANGLE PHASE where students can be observed untangling themselves and attempting to evacuate the bus. chool Bus Rollover Types Using the NASS-CDS System12 There are multiple types of school bus rollovers with conventional Type C or D school buses each type having seven observable phases described above. Among those rollover collision types are:

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(A) The Trip-over where the lateral motion of the school bus is suddenly slowed or stopped inducing a rollover. The opposing force

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overturns as a consequence. The rollover must occur in close proximity to the object from which it is deflected.

(F) Turn-overs occur when centrifugal forces from a sharp turn, hard steering and evasive action where there is no indication of surface furrowing, or gouging at the point of impact.

(G) Collision with another vehicle of similar or greater weight than a 20,000+ lbs. school bus may result in a rollover of one or both vehicles.

(H) The Climb-over occurs when a bus climbs up and over a fixed object such as a guardrail, Jersey barrier, etc. The bus must roll over onto the opposite side from which it approached the guardrail or barrier.

Trip-over. 3.21.2006 Convoy, OH. Bus Driver Avoids Hitting a Deer in the Road and Overturns the Bus13 may be produced by a curb, a pothole or pavement that the bus wheels dig into. The most common precipitating event is the driver avoiding a collision with another vehicle, a person or an animal.

The bus driver oversteers and losing steering control sets into motion bus instability resulting in a bus rollover.

(B) The Slide-over occurs frequently on bus route in suburban and rural areas that causes the bus to slide and tip over after moving off the pavement onto the shoulder at a student pickup site. Many bus stops are on unpaved, sloped, grassy or muddy surfaces.

C) Fall-overs happen when the surface on which the vehicle is traveling slopes downward in the direction of bus movement so that the center of gravity becomes outboard of its wheels. The distinction between fall-overs and turn-overs is the presence of a negative slope.

(D)The Flip-over occurs when the bus is rotated around its longitudinal axis by a ramp-like object such as a guardrail or the back slope of a ditch.

(E) Bounce-over is when a bus rebounds off a fixed object and

(I) End-over-end Rollovers. The last major type of bus rollover is the End-over-end rollover when the bus rolls primarily about its lateral axis. he Roll Cage/Roof Crush Factor While several Federal Motor Vehicle Safety Standards (FMVSS) address school bus passenger protection the effect of rollovers on unbelted students has not been adequately researched. Type C (conventional) and D school buses (transit type) prevents or reduces roof crush intrusions. That same “roll cage” construction does not assure rollover collision safety for those unbelted passengers within compared to the relatively “minor” injuries sustained by students who are belted in. Those displaced and unbelted passengers have two to three feet further to fall or to be thrown “upward” onto the ceiling in a 180 degree rollover. They may be thrown laterally across the full width of the bus interior or longitudinally forward or backward to either end of the bus. The roll cage/roof crush factor also applies with others who experience multiple and progressive displacements during complex rollovers greater than 180 degrees. The roll cage/roof crush factor must be further studied using videotape or digital footage of bus collision events with unbelted passengers to confirm or deny the extent of the injuries imposed during the THROW PHASE.

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Rollover Passenger Ejections from the Compartmentalization Envelope (CE). Rollovers cause multiple collisions between adjacent or across-the-aisle bus seatmates. Students are ejected out of the CE forward or rearward over the high-backed seats and out of the compartmentalization envelope16. High-backed well-padded school bus seats reduce passenger-to-passenger collisions only in frontal or rear collision events but may be irrelevant in lateral or rollover collisions. Bus interior surfaces are not well-padded, energy-absorbing landing zones compared to the interiors of smaller passenger vehicles that provide less head-to-roof crush distances and have less lateral displacement for their lap-belt secured passengers. Vidoanalytics should be able to determine from collision videotapes the relative probability of increased school bus passenger injuries with those removed from their compartmentalization envelope. he People Slosh Factor Accident reconstruction formulas account for liquid or unstable cargo displacement and instability but may not account apply to school bus “people slosh.” People slosh within the bus for those who are not seat belted destabilize a school bus leading to, or being the cause of, bus rollovers. School buses are well known for their relatively high center

T Fall-over. 5.20.2003 Siloam Springs, AR14.

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cessed 8.27.2008.

2. National Association of Pupil Transportation. School Bus Information Clearinghouse. School Bus Facts. http://sbi.elitedecision. com/index.php?option=com_content&task=view&id=12&Itemid =27 accessed 8.27.2008 3. Jennifer McGeehan, MPH, Joseph L. Annest, PhD, Madhavi Vajani, MPH, Marilyn J. Bull, MD, Phyllis E. Agran, MD, MPH and Gary A. Smith, MD, DrPH. School Bus–Related Injuries Among Children and Teenagers in the United States, 2001-2003.

PEDIATRICS Vol. 118 No. 5 November 2006, pp. 1978-1984.

4. Phyllis E. Agran and the NASDPTS article Passenger Crash Protection in School Buses receives an apology regarding she and her colleagues being misquoted. http://www.nasdpts.org/paperCrashProtect.html accessed 8.27.2008.

5. P.C. Lapner,M.D., Duong Nuygen, M.D., and Mervyn Letts, M.D. Analysis of a school bus collision: Mechanism of injury in the unrestrained child. Canadian Journal of Surgeons, Vol.6 No.4, August, 2003. 269-272.

6. Turner, Ray 2008A. School Bus Occupant Displacement Videoanalysis Using Photogrammetry and Reverse Projection. Concept Paper. San Antonio: School Bus Accident Reconstruction, LLC.

Climb-over. 11.22.06 Huntsville, AL with seven fatalities15. of gravity around which bus chassis and frame and the passengers within move, spin, roll or tumble. Passengers seated all on one side of the bus change the bus center of gravity to add lateral instability to vehicle forward motion. Unbalanced student “loading” increases the propensity for bus rollovers. Passengers thrown to one side inside the bus also add to lateral instability and the energy needed to make the bus turnover more. Videoanalytics may provide a better understanding of the changes in bus center of gravity with passenger movements or displacements. sing Static Stability Factor and School Bus Center of Gravity together to Better Understand Bus Rollovers School buses (Type A, B, C or D) have always had a high center of gravity, the propensity to roll over at least one quarter turn and the concomitant high Static Stability Factors (SSF) values for each school bus type. The center of gravity height increases on many school buses as the weight of the student load increases thereby increasing the rollover risk. SSF is the ratio between one-half of the track width and the height of the center of gravity above the road. The SSF rating system has not been applied to school buses as it has been to passenger cars, SUVs, light trucks and other vehicles. School districts should be provided with SSF and Center of Gravity Height as a means to determine rollover risk factors when purchasing school buses. The SSF rating system ranges from one to five stars where a five star rating means that the vehicle has a less than 10% risk for a rollover down to 1 star vehicles rated to have a greater than 40% risk of rollover. School bus manufacturers should publish their COG/SSF ratings and make them more understandable to the general public and to school officials who make bus purchasing decisions based on the quantifiable bus safety value differences between bus makes, models and years.

When this information is available that is in part the result of videoanalytics applied to school buses then we as a Nation will have begun to solve the puzzle of school bus rollovers.

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eferences 1. National Association of State Directors of Pupil Transportation Services. History of School Bus Safety -- Why Are School Buses Built as They Are? A Position Paper.

February, 2002. http://www.nasdpts.org/paperSHistor.html ac-

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7. Turner, Ray. 2008B. Forensic Videoanalysis Using Close-Range Photogrammetry and Reverse Projection with Mutiplexed School Bus Onboard Video. San Antonio: School Bus Accident Reconstruction, LLC. Concept Paper. San Antonio: White Buffalo Press.

7. Turner, Ray. 2008B. 8. Turner, Ray. 2008.

9. E. A. Moffat. Overview of Rollover Crashes. Passenger Car Rollover TOPTEC. Cause & Rollover Prevention. San Diego, CA. January 21-22, 1999.

10. CBS 60 Minutes II – Rollovers. http://www.collinsaav.com/ cbs_60minutes.htm, accessed 8.27.2008.

11. 12.11.1999. Cotulla, TX. Cotulla Independent School District. Personal Communication. See also San Antonio Express News 12.12.1999.

12. National Automotive Sampling System(NASS). http://wwwnass.nhtsa.dot.gov/BIN/nassCaseList.EXE/SETFILTER accessed on 8.27.2008.

13. Beth L. Jokinen and J.D. Bruewer. 3.21.2006 Convoy, OH. Seat belts prove helpful in bus accident. Freedom Communications, Inc. www.limaohio.com.

14. Arkansas State Police. Siloam Springs, AR. 5.20.2003. Accident Reconstruction Document.

15. Alabama High School, Community Mourns Loss of 4 Killed in Bus Crash. Wednesday, November 22, 2006. Associated Press. http://www.foxnews.com/story/0,2933,231341,00.html accessed on 8.27.20008.

16. Turner, Ray. 2007. Compartmentalization Compromised. Collision. The International Compendium for Crash Research, Vol 2.No.1, 28-33, accessed 8.27.2008.

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Bus Braking Tests With and Without ABS D. Vangi

A. Virga

Department of Mechanics and Industrial Technologies University of Florence - Italy

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ntroduction During studies carried out to reconstruct a traffic accident, experimental tests were carried out to evaluate the braking performance of an intercity bus, both with and without ABS. This verification was deemed necessary in consideration of the seriousness of the accident, as well as to determine possible responsibility, especially as regards the speed at which the bus was being driven.

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Since the very first vehicles equipped with ABS (Antilock Braking System) came into use, the effect of their negative acceleration capacity as well as the significance of the skidmarks left by the tires have been questions open to study.

The technical literature contains numerous studies on the behaviour of automobiles and abundant data on their braking performance.

For heavy vehicles instead it is not easy to find data on deceleration values in various conditions. This paper reports experimental data on the performance of a bus in normal operating conditions as well as in the case of failure in the ABS, casting light on the vehicle’s overall behaviour. ctivity The purpose of the experimental testing was that of evaluating braking characteristics and comparing the emergency braking performance of an intercity bus equipped with ABS (Antilock Braking System) and one without such a system.

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Fig. 1 - Texture of pavement at test site.

Fig. 2 - Positioning of ballast.

The vehicle tested was a FIAT IVECO 370S, whose main characteristics are listed in Table 1.

The general scheme of the braking system with which the bus is equipped may be briefly summarized as follows: - Service brake and emergency brake: 1st axle pneumatic/hydraulic brake; 2nd axle air brake; foot brake on all 4 wheels. There are two independent circuits, one for the front axle and one for the rear axle.

- Parking brake: mechanical on the rear wheels. The air brake is a hand brake with emergency blocking device.

- TELMA FOCAL electromagnetic decelerator, controlled by service brake pedal or by a separate control. In place of the pedal control, a dashboard switch can be used.

The vehicle was decelerated to the maximum of its capability from a speed of about 75 km/h to a full stop. The tests were conducted on dry asphalt, of average macro and micro texture (Fig. 1), on a road having longitudinal slope of 2% and transverse slope of 3%.

To simulate real accident conditions as faithfully as possible, the bus was loaded with about 2500 kg of ballast, distributed along the seats (Fig. 2).

The front wheels were fitted with new tires and the rear ones with recapped tires and thread height ranging from 11 mm to 14 mm (as was determined at the time of the accident by Police Officers).

The tests were conducted according to the following procedure: - signal acquisition begins with the vehicle at a stop; - gradual acceleration up to the established speed; Table 1 – Main characteristics of the vehicle under test.

Vehicle Fiat Iveco 370S

Length 12 m

Width 2.5 m

Wheelbase

Tires

Displacement

Drive

Mass

6.29 m

Front 295/80R22,5 Rear 295/80R22,5 (twin)

13798 cc Diesel

Rear-wheel

11500 kg + 2500 kg (ballast)

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only those obtained with the accelerometer are reported below.

- brakes applied to the maximum of the system’s capacity;

esults For each test, processing of the acceleration signals (Figs.

3 and 4) acquired by the accelerometer in both directions yielded the following results:

- full arrest of the vehicle; - signal acquisition ends.

The first four tests were conducted with the ABS in operation, followed by three tests with the ABS switched off. After each test the skidmarks left by the tires on the road surface were measured.

The kinematic quantities were measured using two instruments: - biaxial accelerometer, with acquisition frequency of 61 Hz; - GPS based system, with acquisition frequency of 10 Hz. The first instrument, designed and manufactured by AGE Scientific®, is based on the MXD7202 biaxial accelerometer chip produced

by Memsic Inc.®. It acquires accelerations at a frequency of 61 Hz and is equipped with 10 static memory banks downloadable by personal computer through a serial port. The instrument was fixed to the pavement in a central position sufficiently near the vehicle’s centre of gravity, with one axis coinciding with the direction of motion and the other extending along the vertical.

For each test the longitudinal and vertical acceleration signals were acquired. The vertical acceleration was used to evaluate and correct any slope in the positioning of the instruments and to compensate for the pitching effect during braking.

The second measurement instrument, called PerfomanceBox, is based on the UBOX system manufactured by Racelogic®. It measures position at the frequency of 10 Hz by means of the GPS system; velocity is determined by the Doppler effect on the GPS waves. The position in time and the other quantities (velocity, acceleration, etc.) are directly furnished by this system. Since the values measured by the two instruments were basically the same,

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- acceleration curve in the direction of motion in relation to time; - by integrating backwards with respect to time the acceleration signal in the direction of motion starting from the final position of rest, the speed curve in relation to time was obtained. The moment when braking begins was assumed to be the one in which longitudinal acceleration reaches the value of –0.5 m/s². This assumption was made to nullify the possible difference in pressure exerted on the brake pedal by the driver in the different tests. If for some reason, in fact, the driver reduces the braking force on the pedal, there will be a certain segment (in both distance and time) covered at constant negative acceleration; the mean overall acceleration value will be affected by this circumstance, lowering the real braking capacity of the vehicle.

- by integrating the speed signal with respect to time, the distance travelled during braking in relation to time was determined; - mean value of acceleration in respect to time; - mean value of acceleration in respect to space.

By way of example, the values of acceleration, speed and distance measured with the GPS in Test #2 are shown in Fig. 5.

The results obtained are reported in Table 2. Specifically, the mean values of acceleration, the braking start velocities (trigger at –0.5 m/s²), the braking length and the length of the skidmarks left on the pavement are shown, as measured by the two instruments.

Note that, with both of the instruments utilised, the tests with

Fig. 3 – Acceleration diagrams for tests with ABS, instrument 1 (accelerometer).

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Fig. 4 – Acceleration diagrams for tests without ABS, instrument 1 (accelerometer). (above) Fig. 5 – Test #2, braking with ABS, instrument 2 (GPS).(below)

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Test description

Braking speed (km/h)

Stopping length (m)

#

accelerometer

accelerometer

type

GPS

Mean acceleration (m/s²)

GPS

skidmarks

accelerometer

GPS

1

with ABS

77.0

73.6

37.9

36.0

34.4

6.0

5.9

2

with ABS

67.0

67.0

26.4

28.5

19.1

6.6

6.4

3

with ABS

73.7

72.7

39.2

39.8

27.5

5.4

5.2

4

with ABS

75.4

74.2

38.1

40.0

28.5

5.8

5.6

5

without ABS

74.0

n.a.

40.5

n.a.

32.3

5.2

n.a.

6

without ABS

77.4

74.1

42.9

40.3

37.5

5.4

5.1

7

without ABS

71.2

68.0

37.3

36.1

31.1

5.3

5.0

Table 2 – Summary of results.

Mean (m/s²)

Minimum (m/s²)

with ABS

without ABS

with ABS

Accelerometer

6.0

5.3

5.4

GPS

5.8

5.1

5.2

Instruments' mean

5.9

5.2

5.3

5.1

Maximum (m/s²)

without ABS

with ABS

without ABS

5.2

6.6

5.4

5.0

6.4

5.1

6.5

5.3

Table 3 – Mean, minimum and maximum values of mean negative acceleration measured in the various tests with the two instruments.

Test description # type Test 1 with ABS Test 2 with ABS Test 3 with ABS Test 4 with ABS Test 5 without ABS Test 6 without ABS Test 7 without ABS

Max. acceleration (m/s²) 7.27 8.22 8.18 7.77 8.54 8.46 7.77

Mean values (m/s²)

7.86

8.25

Table 4 – Peak negative acceleration values during the first stage of braking.

ABS are characterised by notable scattering, while those without ABS show very similar results.

Considering the mean values of the two instruments, the difference in the negative acceleration found in the tests with and without ABS is limited to 0.7 m/s² (5.9 as compared to 5.2: Table 3). The difference between the worst and the best measurement is instead 1.6 m/s².

Peak negative acceleration values found in the various tests are shown in Table 4. Note that these values are lower for the tests with ABS, due to the effect of the antilock braking system which, relative to tire slip, prevents negative acceleration from coming too close to the limit value to avoid the risk of excessive or inefficient skidding. iscussion Tests with ABS The tests conducted with the ABS in operation show a typical behaviour: acceleration increases in virtually linear manner during the first 0.4 – 0.5 s, until reaching peak acceleration. Then the ABS device intervenes, keeping the wheels from locking, and the braking force is modulated (Fig. 3). After some oscillation, with a duration of about 1.2 – 1.5 s, acceleration stabilizes at a value quite close to that of peak, when the velocity has

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dropped to around 35 km/h. The stabilized acceleration value is substantially higher than the one corresponding to sliding friction measured in the tests without ABS. During modulation of braking, the mean negative acceleration value is lower than the one found in the final stage of stabilized braking. This is due to the fact that in this stage braking torque is released by the ABS and vertical oscillations of the vehicle occur, as can be seen on the vertical acceleration plots (see for example Fig. 6). When the stabilized braking stage is reached, these oscillations cease.

The skidmarks observed were left by the left front wheel alone, indicating a slight unbalance in the ABS braking system. Modulation of braking results in a typical skidmark pattern (see Fig. 7) with alternating sections of pronounced marks and barely visible marks.

These alternating skidmarks are visible up to the vehicle’s final resting position, indicating that the ABS has exerted modulation on this wheel for the entire length of braking, including the portion of stabilized braking. ests without ABS In the tests without ABS (Fig. 4), acceleration increases in virtually linear manner during the first 0.4 – 0.5 s and after having reached the maximum peak drastically decreases, remaining at a value that varies slightly in relation to speed.

T

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Fig. 6 – Test #4, braking with ABS, instrument 1 (accelerometer).

During this stage and up to the almost full stop of the vehicle, no evident oscillations in the acceleration values are observed (see vertical acceleration in Fig. 8).

During this stage, acceleration is proportional to the coefficient of sliding friction, making it possible, for example, to evaluate its behaviour on the basis of speed (Fig. 9). The peak value reached in the initial braking stage is in relation to the maximum friction coefficient, usually attained in the case of limited skidding slip (about 20%).

The skidmarks left during braking appear continuous and well marked, left by all of the wheels to different degrees (see Fig. 10). From comparison between the length of the marks measured on the ground and the distance covered during braking, it can be seen that in two cases out of three, the skidmarks begin immediately before acceleration reaches its peak value and thus before the locking of the wheels. In the literature [5] on braking in automobiles, the skidmarks are generally observed immediately after the maximum acceleration peak, and not before.

However, tests on heavy vehicles [5] have shown that the braking skidmarks appear earlier than with automobiles.

It should be taken into consideration that the tests were performed in sequence, so that the braking system was undoubtedly subjected to fatigue, and the tires to progressive wear. Note that in the case of a braking system and tires not already stressed by previous braking, the skidmarks with ABS are barely visible. This is not the case with subsequent braking manoeuvres, still with ABS, in which the skidmarks appear progressively heavier and more visible. However, the characteristic discontinuity indicative of the intervention of the ABS can still be observed. alculating velocity based on skidmarks Shown in Tab. 5 is the initial speed of each test compared with speed calculated on the basis of the balance between kinetic energy dissipated and work of the friction forces, according to the equation:

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Fig. 7 – Skidmarks produced by braking with ABS (Test #2).

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Fig. 8 – Test #6, braking without ABS, instrument 1 (accelerometer). (above) Fig. 9 – Behaviour of mean sliding friction coefficient with speed. (below)

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Test description

Mean acceleration

Test speed

Speed from skidmarks

Speed from skidmarks*

#

type

(m/s²)

(km/h)

(km/h)

(km/h)

Test 1

with ABS

6.0

77.0

73.1

76.7

Test 2

with ABS

6.6

67.0

57.2

60.0

Test 3

with ABS

5.4

73.7

62.0

65.1

Test 4

with ABS

5.8

75.4

65.5

68.7

Test 5

without ABS

5.2

74.0

66.0

69.2

Test 6

without ABS

5.4

77.4

72.5

76.0

Test 7

without ABS

5.3

71.2

65.4

68.6

* Skidmark length increased by 10% Tab. 5 – Speed calculation from skidmarks.

with x the length of the skidmark, and gf equal to the mean acceleration obtained in each test. Since in many cases, to take account of the fact that negative acceleration starts before any skidmarks are produced on the ground, a virtual length is employed, obtained by incrementing the length of the skidmarks by 10%; this calculation also appears in the table.

It can be noted that, especially in the case of braking with ABS, even when the skidmarks are clearly visible, they must be measured very carefully (especially when it is hard to determine their starting point, which is sometimes very faint) and that speed values estimated starting from the length of the skidmarks can be subject to an error of several km/h. Consequently, the speed at start of braking can be estimated on the basis of length of the skidmarks only in cases where such uncertainty is acceptable for the purposes of the case in question.

ibliography [1] Vangi D., Virga A., “Evaluation of emergency braking deceleration for accident reconstruction”, Vehicle System Dynamics, Vol. 45, No. 10, October 2007,

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895-910 [2] Oppenheimer P., “Comparing stopping capability of cars with and without Antilock Braking Systems (ABS)”, Technical paper 880324, SAE 1988 Metz L. D., Ruhl R. L., “Skidmark signatures of ABS[3] Equipped passenger cars”, Technical paper 900106, SAE 1990 Robinson E. L., “Analysis of accelerometer data for use in [4] skid-stop calculations”, Technical paper 940918, SAE 1994 Reed W. S., Taner Keskin A., “Vehicular deceleration and [5] its relationship to friction”, Technical paper 890736, SAE 1989

onclusions This paper presents the results of experimental braking tests performed on a bus used for civil transportation, within the sphere of reconstructing the mechanics of a traffic ac-

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cident.

The study provides quantitative indications of the performance of a bus both in normal operating conditions and in those of failure in the ABS system, useful for reconstructing road accidents. Such data, generally available for cars, are in fact harder to find for heavy vehicles.

The braking performance of the vehicle with ABS in operation was clearly better than in the condition of disablement of the assisted braking system. The results obtained by comparing the data in the different test conditions, in terms of both mean acceleration and braking distance, are presented. The case studied has shown the level of uncertainty in calculating the initial velocity based on measurement of the skidmarks left on the pavement. The experimental data presented were verified with dual instrumentation for the purpose of reducing the contribution of systematic uncertainties of the two measurement and data processing systems. The test methods, developed according to an established protocol, were designed to curtail accidental uncertainty such as variations in the driver’s behaviour and in local conditions; accordingly, the variability observed in relation to acceleration may be attributed to the precise conditions of the vehicle in each individual test. The mean value obtained in the different tests thus reflects the real performance of the vehicle, with only minimal contributions of uncertainty produced by the boundary conditions.

Fig. 10 – Skidmarks produced by braking without ABS (Test #7). www.collisionpublishing.com

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Hydroplaning

The effect of Water on the Roadway Elvin Aycock PE, PLS, ACTAR

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ntroduction Hydroplaning is a hazardous condition resulting in many accidents each year. Hydroplaning occurs when tires move too fast across a wet pavement - so fast that they do not have sufficient time to channel the water away from the center of the tire. The results are that the tires are lifted by the water away from the road and all traction is lost.

Once all traction is lost, the driver has no control of his vehicle.

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This report will discuss a hydroplaning accident on an interstate highway in south Georgia. It was suspected that the depth of water on the roadway surface was sufficiently deep to contribute to the hydroplaning of the vehicle.

Two causes of the accumulation of the water at this location will be discussed in this paper.

This is the build-up of the water depth as accumulated water traverses the pavement surface. At the crown of the roadway, the depth of the water is essentially zero. As rain continues, the water at the crown begins to move downgrade and progressively gets deeper as it crosses the pavement surface. Water that

travels across several lanes can accumulate to sufficient depth to cause hydroplaning.

The second cause of a potential water problem on roadways is created by water flow constricted by curb and gutter sections.

When a section of curb and gutter is constructed in a manner that causes the water to flow into the travel lanes, water builds up along the curb and gutter section and spread out into the travel lanes. If the spread of the water is sufficiently deep in the travel lanes, hydroplaning can occur.

Also, found in the area of the accident was a catch basin completely filled with sand and silt. The structure was completely clogged and hidden by grass. No water could get into the catch basin. The effect of the clogged catch basin spreading water into the travel lanes was also investigated.

In 1966 Arthur D. Little stated in his report, The State of the Art of Traffic Safety that: “Skidding has emerged as a factor of major importance to the overall traffic safety problem, and the surface characteristic of principal concern appear to be the frictional properties of the

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gutters are rarely constructed along a high speed highway. Curbs create a potential for water spreading into the travel lanes.

Longitudinal Slopes: The longitudinal slope of the roadway is the slope parallel to the direction of the traffic flow. The American Association of State Highway Traffic Officials (AASHTO) Policy on Geometric Design provides safe, acceptable minimum values for roadway longitudinal slopes. Longitudinal slopes are the vertical curves along the roadway. It is the positive increase in the grade as the roadway rises and the decrease in the grade as the roadway falls to the sag in the vertical curve. The sags in vertical curves are sensitive points along roadways where water can accumulate.

Figure 1: A view of the accident site. The on-ramp is shown on the right of the photograph road surface, both wet and dry. While there is evidence that skidding is involved in a high proportion of accidents, current methods of reporting accidents are believed to be inadequate to determine the true extent of this factor. Since skidding occurs in many accidents, either before or after braking, it is generally overlooked as a cause in most accident reports. There is a tendency to list more obvious causes such as failure to negotiate the curve or lost control of car which are in fact not causes but results; therefore, no accurate record is maintained of the kind and the degree of skidding involvement.”

Reporting methods of hydroplaning accidents have changed little since 1966. Reports focus more on the results of the accident rather than on the cause of the accident. The potential of a vehicle skidding increases dramatically as the coefficient of friction between the vehicle tire and the roadway surface decreases. This decrease is often due to water on the roadway surface.

Another factor which contributes to increased depth of roadway water are surfaces worn down because of years of use. Increase in the volume of traffic decreases the life expectancy of a pavement surface. As age and traffic volume increases, the roadway surface aggregates become worn and polished. Worn surfaces on dry or wet pavement increases the likelihood hood of skidding because the friction coefficient of the surface is decreased.

The demand for more highways has increased and more lanes are being added to existing highways. As the number of lanes increase, the travel distance of water increases and the depth of water on roadway increases. The increased depth of the water on the roadway surface increases the potential for hydroplaning.

Cross Slope: Acceptable transverse (or cross slopes) are outlined in the AASHTO Policy for Geometric Design of Highways and Streets. Studies have shown that the greater the cross slope of the pavement, the better the ability of the pavement to discharge storm water. However, as the slope of the cross section becomes excessively steep, the comfort of the driver and his ability to steer his vehicle is affected. Excessive slopes cause vehicles to pull to the downhill side of the slopes. ase Overview In July of 2003 the driver of a 1999 Ford F-350 truck was traveling northbound on Interstate 75 in south Georgia when the vehicle hydroplaned, crossed the median, and collided with a 2001 Honda Odyssey. It was estimated the driver of the Ford was traveling at approximately 70mph, the speed limit.

The weather was cloudy with intermittently scattered rain occurring in the area. The driver of the pickup stated in his report to the police that the rain began to come down in heavy gusts. He was behind a truck/tractor and trailer in the right travel lane. As the rain began, the truck/tractor rig slowed and the driver of the pickup attempted to move to the left lane to pass the truck/tractor and trailer.

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He stated that just as he made the steering maneuver to change lanes he could feel loss of control of his vehicle and his vehicle began to rotate. His pickup truck traveled approximately 300 feet before leaving the left travel lane and entering the median. The pickup truck continued to rotate, crossed the median, entered the southbound roadway and hit the oncoming minivan.

The attorney requested an investigation of the factors causing the loss of control of the pickup truck. He wanted to know if the pickup truck hydroplaned. If the pickup truck hydroplaned, what were the conditions of the roadway at the location in the area of the hydroplaning? An investigation was begun to determine the quantity of storm water and the depth of the storm water on the roadway at the time the pickup truck lost control.

Effective drainage of highway pavements is essential for traffic safety. Water on the roadway surface must be removed as quickly as practical and be directed in such a manner as to prevent localized accumulation of water pools.

The accumulation of water on the pavement is a function of many factors. This treatise will focus on the factors affecting the depth of the water on the roadway and the methods to determine the water depth.

Surface Drainage: Storm water falling on a sloped pavement surface forms a thin layer of water that increases in depth as the water flows across the pavement. The slope of the pavement is a combination of the longitudinal slope and the traverse slope. These slopes dictate the length of the travel path of the water crossing the pavement surface.

Paved Shoulders: Most high-speed highways have wide paved shoulders of approximately ten feet with the paved shoulder steepened to quickly remove the water from the roadway. Curbs and 80 Collision

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Figure 2: Notations have been made showing theon-ramp, the gore area, the curb drain, and the area of water accumulation

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Roadway Conditions: The roadway was a concrete surfaced interstate highway with two northbound lanes and two southbound lanes. A forty-foot grassed median separated the travel lanes. The inside shoulder was a four-foot asphalt pavement and the outside shoulder was a 10-foot asphalt shoulder. The construction plans showed the cross-slope of the travel lanes of the roadway to be one percent. The crown of the roadway was the inside fog line and water traveled across two lanes, onto the asphalt shoulder, and into a grassed ditch.

A curb and gutter section was constructed along a 90-foot section of the roadway adjacent to the outside travel lane at the northbound on-ramp beginning south of the on-ramp. The curb and gutter turned and ran along the on-ramp for approximately 260 feet. The curb and gutter section had a 24-inch gutter with a sixinch curb. The gutter abutted the fog line of the right travel lane of the roadway. No inlet was installed in the gutter to collect the water and the water emptied into the lane of the on-ramp.

A catch basin was found in the curb of the on-ramp which was designed to intercept the water carried by the curb and gutter. At the time of the accident, the opening of the catch basin was completely filled with silt and sand and covered with grass. No water could get into the catch basin.

tor control and will increase the stopping distance of the vehicle.

Full dynamic hydroplaning occurs when the interface between the tire and the pavement is unable to remove the water fast enough to allow the tire to maintain contact with the pavement.

Factors which contribute to hydroplaning are lack of proper tire tread depth, low tire pressure, high speeds, well-worn pavement texture, water accumulations, and pooling of water created by conditions of the roadway which cause poor drainage of the roadway pavement.

Sections of roadways are sometimes inadvertently constructed with irregularities in the longitudinal and lateral slopes which cause the road surface to pool water. Heavy traffic will cause ruts in the travel paths and act to channel water along the roadway.

Several highway situations often produce the potential for hydroplaning. These include (1) the bottom of a sag in a vertical curve, (2) at the horizontal curve transitions, and (3) in ruts caused by vehicle tires.

The Hydraulic Engineering Circular No. 22, Second Edition produced by the U. S. Department of Transportation and the Federal Highway Administration, page 4-4 states: “In problem areas, hydroplaning may be reduced by the following: 1. Design the highway geometries to reduce the drainage path lengths of the water flowing over the pavement. This will reduce flow build-up.

2. Increase the pavement surface texture depth by such methods as grooving of portland cement concrete. An increase of pavement surface texture will increase the drainage capacity at the tire pavement interface.

Figure 3: The catch basin at the time of the accident. Silt and grass had filled the curb opening preventing water from entering the basin The longitudinal slope of the northbound travel lanes were downgrade at 0.6278 percent. The divided highway was designed to slope at one inch per eight feet from the inside edge of the left travel lane to the fog line on the outside travel lane. The lateral cross slope varied from 0.92% to 0.98%. The design of the pavement surface directed water in a northeasterly direction across the pavement.

Hydroplaning: Hydroplaning by vehicles is a phenomenon characterized by complete loss of directional control when the tires are lifted off the pavement surface by water. The vehicle must be moving fast enough to ride up on a thin layer of water. When a rolling tire encounters a film of water on the roadway, the water is passed through the tire tread and through the surface texture of the pavement. At a point the tire and the pavement are unable to displace all the water and the tire rides up on the water. Once the tire lifts off the pavement and contact with the pavement is lost, the vehicle tire hydroplanes.

There are two types of hydroplaning, partial hydroplaning and full dynamic hydroplaning. Partial hydroplaning occurs when the tire loses partial contact with the pavement surface. The reduction in contact pressures results in a lowering of the friction of coefficient between the tire and the pavement. This results in the loss of opera-

3.The use of open graded asphaltic pavements has been shown to greatly reduce the hydroplaning potential of the road surface. This reduction is due to the ability of the water to be forced through the pavement under the tire. This releases any hydrodynamic pressures that are created and reduces the potential for the tire to hydroplane.

4.The use of drainage structures along the roadway to capture the flow of water over the pavement will reduce the thickness of the film of water and reduce the hydroplaning potential of the roadway surface.

Tread depth, weight of a vehicle, tire pressure, depth of water, and condition of the road surface all play a part in determining the speed at which a tire will begin to hydroplane.

Water Depth by the Drop Method: In the case of this accident it was determined that two potential reasons accounted for the accumulation of the water on the roadway and contributed to the hydroplaning of the vehicle. The first was the increasing depth of the water as the water traversed across the travel lanes from the inside paved shoulder of the roadway to the outside edge of the outside travel lane. Second, the water which traveled to the curb and gutter backed out into the travel lanes. As the depth of the water in the gutter increased, the spread of the water into the travel lanes increased. Both factors required a detail survey of the roadway to calculate the depth of the water on the roadway.

The decision was made to investigate 1,400 linear feet of the northbound lane since the driver was unable to identify the location of the initial loss of control of his vehicle.

Roadway Survey: The survey crew used a differential level to obtain elevations within the travel lanes and the paved shoulders. A rectangular grid was laid out on the roadway at intervals of 10 feet longitudinally and 6 feet laterally. Elevations were taken at each

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Figure 4: The survey grid was laid out at 10 ft intervals along the roadway and 6 ft intervals across the roadway. Contour lines were created from the spot elevations

Figure 6: The path of the raindrop follows the downslope and runs perpendicular to the contour lines. the raindrop path is more than 12 ft longer than the design path

grid point and plotted on a survey map. Contour lines were generated from the survey information.

for a distance of approximately 300 feet. The longitudinal slope of the roadway was found to vary from 0.620 percent and 0.6278 percent. The cross slope or transverse slope varied from 0.0 percent to 0.98 percent.

A total station was used to survey the roadway outside of the travel lanes and the paved shoulders. Approximately 1,400 foot-section of the northbound lanes was surveyed to determine areas of irregularities.

The survey showed the pavement to be irregular along both lanes

The direction of the flow of water can be determined from contour maps. Water travels perpendicular to contour lines. Surfaces with contour lines which are straight and parallel will direct the water in a straight line. Since the shortest distance between two points is a straight line, the water removal is most efficient with surfaces of straight and regular contours.

The depth of water increases as the length of the path of the water increases. Because the slope of the roadway varied, it caused the length of the travel path of the water to increase.

The increased length of travel caused the depth of the water to increase as the water flowed across the road surface.

Figure 6 shows the contour map drawn as the roadway was constructed. This information was obtained from the survey spot elevation shots. The pavement was constructed with an irregular cross slope as shown by the contour lines. The path of the raindrop has a longer travel distance to travel to get off the pavement and therefore the depth of the water becomes deeper than designed.

Texture Depth Testing: Surface texture depth was performed by the National Center for Asphalt Technology (NCAT). Measurements were taken in both northbound travel lanes in the area of investigation. Three measurements for the surface texture were taken transversely across each lane with one test in each wheel path and one test between the wheel path. Two readings were taken at each location and the results averaged and reported as one test.

Figure 5: The contours shown are as per the design plans. The path of the raindrop runs perpendicular to the contour lines and is the shortest path across the pavement 82 Collision

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The Circular Texture Meter (CT Meter) (ASTM E2157) was used for conducting texture measurements of pavement surfaces. Pavement friction during wet conditions affects the pavement s ability to channel water across its surface and the friction of a pavement surface is a function of the surface texture. Microtexture provides a gritty surface to penetrate thin water films and produce good fric-

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tional resistance between the tire and the pavement. Macrotexture provides drainage channels for water expulsion between the tire and the pavement thus allowing better tire contact with the pavement to improve frictional resistance and prevent hydroplaning.

Pavement skid resistance is tied to surface texture which is the macrotexture of the surface. Two methods for measuring macrotexture are the sand patch test (ASTM E 965) and the laser method. These methods correlate the macrotexture with skid resistance. The simplest surface texture measurement is the sand patch test. The test is carried out on a dry pavement surface by pouring a known quantity of graded sand onto the surface and spreading it in a circular pattern with a straightedge. As the sand is spread, it fills the low spots in the pavement surface. When the sand cannot be spread any further, the diameter of the resulting circle is measured. This diameter of the circle is correlated to an average texture depth, which can be correlated to skid resistance.

The laser method consists of a stationary image processing instrument and an image is taken from a vehicle moving at normal travel speeds. One particular device, the Road Surface Analyzer (ROSAN), a series of non contact pavement surface texture measurement devices, has been developed by the FHWA’s Turner Fairbanks Research Center Pavement Surface Analysis Laboratory. Neither the sand patch test nor the dynamic laser method were used in this investigation.

Length of Travel of Water Across the Pavement: Survey topographic maps were used to create contour maps of the roadway surface.

The path of a drop of water crossing the roadway could be determined and therefore the length of the path of the water could be measured at any point on the map.

The interstate roadway consists of two northbound concrete lanes with a 10-foot outside asphalt shoulders to direct the water to a ditch. The four-foot inside asphalt shoulders moved water to the median. A forty-foot grassed median separates the north and southbound lanes. The design plans called for a one percent cross slope and a 0.6278 percent longitudinal slope in the area under investigation. The roadway was designed to drain the storm water from the fog line on the inside lane across the roadway to the asphalt shoulder at the outside lane.

A field survey of cross sections of the roadway was performed to determine the direction and slope of the pavement. The cross sections were taken at 10-foot intervals along the roadway. Across the roadway elevations were determined at the inside fog line, the centerline of the inside lane, the centerline between the two travel lanes, the centerline of the outside lane, and the fog line on the outside lane. The elevations of the asphalt shoulders were surveyed to document the percent slope of each shoulder.

It was found that the slopes of the cross sections were not uniform across the roadway. The slope from the inside fog line to the centerline of the inside lane differed from the centerline of the inside lane to the center of the two travel lanes. Also, the slope from the center of the two travel lanes to the centerline of the outside lane differed from the centerline of the outside lane to the fog line of the outside lane. See Figure 6. The difference in the water travel path length in Figure 5 vs. Figure 6 is more than 12 feet.

The changing slope of the roadway caused water to travel slower on the flat slopes and faster on the steeper slopes. The water would rush across the steeper slopes and slow in the flatter areas much like water traveling in a supercritical flow and suddenly moving onto a flat area becoming subcritical flow. This created areas in the roadway which were deeper than other areas. The variable slopes created a path of travel of the water to be longer than if the roadway had consistent regular grades. The longer the travel path across the road surface, the deeper the water gets before it exits onto the shoulder of the roadway.

By knowing the rainfall intensity, texture, drainage path length, and cross slope of the pavement, the depth of the water at any point along its path could be determined.

Rainfall Intensity: Rainfall Intensity is the rate at which rain falls.

Intensity is usually stated in inches per hour. The duration of the rainfall is the total rainfall in a particular time period. The size of a storm is described by the number of inches of rainfall together with the duration of the rainfall. Thus, a rainfall event of 2.0 inches over a 24-hour period is different from 2.0 inches of rainfall in an hour. Although both events produced the same rainfall, the latter is more intense.

A return period is the probability of occurrence in any given time frame. It is the average number of years between two rainfall events that equal or exceed a given number of inches over a given duration. A 10-year rainfall event is often defined as a storm of a given rainfall intensity which happens once every 10 years. More accurately a 10-year rainfall event would be described as a 10 percent chance of a particular rainfall happening in any one-year.

Hydrologist use Intensity-Duration-Frequency (IDF) curves to estimate rainfall events. Meteorologists have been keeping data on rainfall for more than 200 years and have produced IDF curves for most of the country.

Rainfall data for our case was provided by a meteorologist. It was determined that the rainfall intensity at the time of the accident would be between one inch and 6.12 inches per hour.

Drop Method Formula: The depth of water on a roadway increases as it flows across the roadway. As the length of the travel path increases, the depth of the water increases.

Two lane roadways have a crown in the middle the pavement and the roadway is sloped to direct the water to each side of the road surface. The length of the path of the drop of water which falls at the centerline of the roadway only has to travel across one lane.

Our roadway under investigation was a four-lane roadway with a divided median. The road surface was sloped to drain water from the inside fog line to the outside fog line and onto a paved shoulder.

The length of the path of a drop of water falling at the inside fog line is dependent on the cross slope and the longitudinal slope.

The method to determine the depth of a drop of water as it traverses the pavement surface is referred to as the Gallaway Method.

Bob M. Gallaway lead the research team that developed the water depth method, sometimes called the drop method. This method uses the length of the travel path, the average roadway texture, the cross slope, the longitudinal slope, and the rainfall intensity to determine the depth of the water at any point on the roadway.

The equation relating water depth to surface texture, the length of the drainage path, the rainfall intensity, and the pavement cross slope is: d = [3.38 x 10-3 (1/T)-0.11 (L)0.43 (I)0.59 (1/S) 0.42] - T where

d = water depth above top of texture (inches), T = Average texture depth (inches), L = Drainage path length (feet), I = Rainfall intensity (inches/hour), S = Cross slope (foot/foot).

This methodology for determining the depth of water across pavements was presented in 1971 in a paper titled: The Effects of Rainfall Intensity, Pavement Cross Slope, Surface Texture, and Drainage Length on Pavement Water Depths, by Bob M. Gallaway, Robert E. Schiller, Jr., and Jerry G. Rose. It was sponsored by the Texas Highway Department in cooperation with the U. S. Department of Transportation and the Federal Highway Administration.

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Average Texture Depth (T): Average texture depth is the depth measured from the lowest elevation of the voids in the pavement to the peaks of the material in the pavement. The average texture depth for our section of concrete roadway was determined using the Circular Texture Meter (CTM). Three measurements for surface texture were taken transversely across each lane with one test in each wheel path and one test between the wheel path. Two readings were taken at each location and the results averaged and reported as one test.

A total of 24 locations was analyzed for texture depth. The average texture depth of the concrete was determined to be 0.651 mm which is 0.024 inches.

Drainage Path Length (L): The length of the travel path is dependent on the route the rain drop follows. Roadways are designed to direct water in a straight path across the pavement. The travel path is lengthened when the cross slope is irregular. The detailed survey of the roadway and the survey map showed contour elevations and the path of a raindrop starting at a given location and traced across the roadway to its exit point from the roadway. Water travels perpendicular to elevation contour lines as discussed above.

Rainfall Intensity (I): Rainfall intensity is the quantity of rain in a give period of time. Most measurements are provided in inches per hour.

The results of the calculations for the depth of the water crossing two travel lanes was presented for the 1-inch per hour rainfall, the 2-inch per hour rainfall, the 4-inch per hour rainfall, and the 6.12inch per hour rainfall. The depth of the water for the 1-inch per hour rainfall intensity at the outside fog line of the outside travel lane averaged 0.05 inches. The depth of the water for the 2-inch per hour rainfall intensity at the outside fog line of the outside travel lane averaged 0.09 inches, the 4-inch per hour rainfall averaged 0.16 inches and the 6.12-inch per hour rainfall averaged 0.20 inches.

Cross Slope (S): A cross slope is the change in elevation over the width of the roadway. Roadways are typically designed using 1/8 inch per foot (0.0104 ft/ft) to 1/4 inch per foot (0.0208 ft/ft). This section of the roadway was designed to have a cross slope of 1/8 inch per foot. The average cross slope was found to vary from 0.0 ft/ft (no slope) to 0.0167 ft/ft.

A longitudinal slope is the change in elevation as measured along the roadway and changes along the direction of travel. A grade which increases along the roadway is positive and a decreasing grade is negative. The section studied was designed for 0.6278 percent or 0.006278 ft/ft.

Water Depth by Gutter Spread: A 90-foot section of 30-inch curb and gutter was constructed along the northbound outside travel lane at the approach to the on-ramp. The curb and gutter section forms a radius and runs along the on-ramp for approximately 90 feet. The curb and gutter section is immediately adjacent to the outside travel lane. No catch basins were installed to capture the water. At the radius the water flows across the on-ramp to a catch basin and the asphalt shoulder.

Typical design standards call for curb and gutter to have a steeper downslope across the gutter face than the slope of the roadway.

Hydraulically this moves the water from the travel lane into the gutter moves faster than if the grade of the gutter was the same as the grade of the cross slope. This captures the water into the gutter channel and removes the water from the roadway. The water captured by the gutter is typically conveyed to a catch basin and removed through a piping system.

The 90-foot section of curb and gutter adjacent to the travel lane was evaluated. The survey data showed the slope of the gutter to be flat or reversed along the 90-foot section. The water that would 84 Collision

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have typically been conveyed to the gutter channel was pushed back onto the roadway.

The Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22, Second Edition, of the U. S. Department of Transportation, the Federal Highway Administration, and the National Highway Institute recommend the use of Table 4-1 (See Figure 7.) for selecting the design storm frequency and design spread for roadways of various design speeds. For highways with speeds greater than 45 mph the design frequency should be the 10-year storm with the spread confined to the shoulder.

Table 4-1 The gutter spread was calculated at three locations along the 90foot length of the curb and gutter. The check points were at the beginning of the curb, the midway point, and at the end point where the curb and gutter emptied onto the on-ramp.

Data required for the determination of gutter spread are drainage areas, the cross slope, the longitudinal slope, the Manning s coefficient, and the rainfall intensity.

T = Width of Gutter Spread, (ft) Q = Flow Rate (cubic feet per second) n = Manning Coefficient Ku = 0.56 in English Units Sx = Cross Slope (ft/ft) SL = Longitudinal Slope (ft/ft) The study point at the midpoint of the 90-foot section of curb and gutter showed the following: Rainfall Intensity

1-inch per hour 2-inch per hour 4-inch per hour 6.12-inch per hour

Spread from the gutter into the Travel Lane 6 feet 7.3 feet 8.6 feet 8.8 feet (10-Year Storm)

Gutter Spread for As-designed Section: An analysis of the gutter spread was made using the design criteria from the construction plans. If the section of curb and gutter has been constructed as designed using a continuation of the cross slope of 1/8 inch per foot, the gutter spread would not have encroached into the travel lane for the 1-year storm. The gutter spread would have encroached one foot into the travel lane for the 2-year storm and 2 feet into the travel lane for the 10-year storm (6.12 inches per hour). onclusion The investigation of the depth of the water in the travel lanes of the roadway was analyzed for two possible causations of the vehicle traffic accident. First, was the hydroplaning caused by the water crossing the roadway or secondly, did the water spread into the travel lanes from the gutter.

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travel lanes of the water flowing across the roadway. It was determined that the roadway travel lanes along this area of the roadway were constructed on an irregular cross and longitudinal slopes. This caused the length of the path of a drop of water crossing the roadway to increase. This increase in the travel path length caused the depth of the water in the travel lanes to be deeper than if the roadway had been constructed as designed.

Additionally, a 90-foot section of curb and gutter was constructed adjacent to the outside travel lane. Also, it was determined that the gutter was constructed without a downslope or at a slope that actually pushed water into the outside travel lane.

Analysis of the clogged catch basin showed the blockage had little effect in the travel lanes and would not have been a factor in the hydroplaning of the pickup truck.

Given the speed of the vehicle, the condition of the tires, and the depth of the water on the roadway, the conditions were conducive to the vehicle hydroplaning.

Figure 8: The water ran along the curb section and created the gutter spread. The curb and gutter was located at the edge of the travel lane and spread into the travel lane Both methods were employed to determine the depth of water in the travel lanes. The drop method is utilized to calculate the cumulative depth of water as it crosses the pavement surface. The gutter spread method determines the depth of water in the travel lanes as the water travels along the curb and gutter and spreads out into the travel lanes.

The drop method was used to calculate the depth of water in the

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Case Problem Solution: A Multifaceted Approach to Collision Analysis Greg Russell, Accident Analysis and Reconstruction Rusty Haight, Collision Safety Institute You’re working a crash where a truck rearends a stopped car then the truck comes to a driver controlled point of rest some distance from the impact. The passenger car moves from impact some distance before striking a third vehicle before coming to rest. You have some CDR crash data to work with but lacking some other independent finding of impact speed, how are you going to adopt that data as reliable in this instance? How are you going to find the truck’s impact/closing velocity?

If you have adopted the conventional wisdom, that a secondary contact or a driver bringing the car to a controlled rest means you “can’t use momentum,” what are you left with?

For the solution to this case problem, we’re going to first examine a crush analysis (which will tie into the CDR report delta-V)

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rush Analysis

Both of the vehicles involved in this collision sustained substantial damage along a horizontal plane (See Figures 3 and 4 with the original case problem statement in this issue). This type of damage profile is well suited for a crush analysis. In a typical crush analysis, after obtaining the crush measurements and the weights of the involved vehicles, the individual stiffness coefficients of the vehicles are calculated or obtained from various published or commercial sources. All of the obtained data is then used to determine the energy that was expended to produce the damage to the vehicles, after which the change in velocity (∆V) that each vehicle experienced may be calculated. Finally, the calculated ∆V along with other data may then be used to determine impact and post impact velocities but one requires stiffness coefficients.

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tiffness Coefficients

Whether the stiffness coefficients for a vehicle are calculated from the NHTSA crash test database or obtained from commercial or public sources, the damage profile of the test vehicle has to be comparable to that of the vehicle involved in the collision. If there is a drastic difference in the damage profile of a vehicle involved in the collision and the profile of the vehicle the stiffness coefficients were obtained from, the stiffness coefficients may not be applicable to the collision being investigated.

The Chevy’s damage profile was reasonably consistent with frontal barrier crash tests conducted for NHTSA. Based on crash test number 4472, it was possible to calculate the stiffness coefficient values for the Chevy. The A stiffness value for the Chevy was determined to be 485lb/in while the B stiffness value was determined to be 143lb/in².

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Figure 5

Figure 6

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Unlike vehicles which sustain frontal damage, there is little crash test data available for vehicles which have sustained rear impact damage. Even if there had been a rear crash test available for the VW, it is unlikely that it would have represented the extent of damage the VW sustained in this collision (see Figure 3 at the case problem statement). Therefore, due to the extent of damage that the VW sustained it is unlikely that any calculated values or rear stiffness values from published or commercial sources would apply to this collision.

The crush profiles for both involved vehicles are seen as diagramed in Figure 6 and Figure 7 depicts their alignment based on damage and an examination of the vehicles themselves.

Figures 8 and 9 show the actual vehicles repositioned as they would have been at impact.

S

tiffness Coefficients from Balancing Forces

The force required to produce the damage to a particular vehicle involved in a collision may be determined independently of any other vehicles involved in the collision. In this collision the force required to produce the damage to the Chevy was determined to be 98,969 pounds. Since the force required to produce the damage to each vehicle as a result of the collision is equal and opposite, the damage force for the VW was also 98,969 pounds.

Having established the collision force for the VW, its value can be used to determine the stiffness coefficients for the VW. In order to calculate the damage force for a vehicle the stiffness coefficients, principal direction of force and damage profile measurements have to be known. In this collision the damage profile of the VW is known, as well as the principal direction of force, yet the stiffness coefficients remain unknown.

When determining stiffness coefficients from crash test data, the following data from the test crash is needed; the weight of the test vehicle, the ∆v experienced by the vehicle in the crash, the width of the damage area, the bo value and the average crush depth. Since there is no reasonably comparable crash test data available for the VW, a model crash test needs to be developed in order to perform the needed calculations. In order to reduce the number of variables needed, already known data may be used for the model. For the vehicle test weight and damage width, the VW’s actual weight and damage width may be used. The bo value is generally an assumed value, typically 5 mph for front and rear impacts. Most barrier tests are conducted at an impact speed of 35 mph, typically producing a change in velocity in the range of 38.5 mph to 42 mph (Based on a restitution range from 0.1 to

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Figure 8

Figure 10

Figure 9

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0.2) . Therefore, it is reasonable to use the same change in velocity for the model crash test that the Chevy experienced in its actual crash test.

For the analysis, one can do hand calculations or develop a simple spreadsheet function to calculate the stiffness coefficients. At this point, Microsoft Excel’s Goal Seeker function may be used to determine what the average crush value for the model would have been. This is done by simply programming into that spreadsheet the identity of the cell containing the calculation for the damage force which should equal 98,969 while the cell containing the value for the average should be changed until the desired result is achieved. To make this a bit easier, Excel contains a function called Goal Seeker which permits the user to produce a specific value in a cell containing a function by changing a single input contained in a separate cell. Through the use of Goal Seeker, an unknown input may be determined based on a desired result. Figure 10 is an example of the Goal Seeker input dialog box. Figure 11 shows an example spreadsheet layout with the Goal Seeker function being used.

The only remaining value needed to calculate the stiffness coefficients for the model is the average crush depth the vehicle would have sustained in the crash test.

When this process is completed using the spreadsheet layout seen in Figure 11, the stiffness values produced for the VW are 112 lb/in for the A value and 46 lb/in² for the B value as seen in Figure 12.

Figure 11

Figure 12

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V

elocity Calculations

Having established the stiffness coefficients for both vehicles, along with the crush measurements the energy expended to produce the damage to each vehicle may be calculated followed by the change in velocity that each vehicle experienced as a result of the collision. These calculations produced a ∆V of approximately 23.4 mph for the Chevy and 39.7 mph for the VW. The calculated change in velocities along with the collision restitution may be used to determine the closing velocity between the two vehicles. Admittedly, in the case the collision restitution will be an estimated value; however, due to the extent of damage that each vehicle sustained, it is reasonable to surmise that restitution value was low. Therefore, assuming a low value for restitution would not be unreasonable. When this is done a closing velocity of approximately 57 mph is produced. Since the VW was stopped on impact, the calculated closing velocity of 57 mph also represents the impact velocity of the Chevy.

Since the VW was stopped on impact, its ∆V of 40 mph also represents it post impact velocity. Finally, the post impact velocity of the Chevy is simply the difference between its impact velocity of 57 mph and its ∆V of 23 mph, or approximately 34 mph.

C

DR data comparison/validation

There are those in within this field who view crush as an unreliable method for determining vehicle velocities akin to the notion that secondary contacts make using momentum “impossible.” However, at times it becomes an invaluable methodology, as in this collision. For those who question the validity of the approach that has been presented or the validity of any crush analysis, this collision did contain a separate validation tool. The CDR data from the ACM included both the Chevy’s speed for the approximate 5 seconds preceding Algorithm Enable and the ∆V experienced as a result of the collision.

The maximum ∆V recorded by ACM was 22.10 mph at 120 milliseconds. This value compares favorably to ∆V value of 23.4 mph produced by the crush analysis.

Starting from AE-5 and moving forward to AE -1, the ACM recorded pre impact speeds for the Chevy of 57 mph slowing to 55 mph. These values also compare favorably to the closing/impact velocity calculation for the Chevy of 57 mph based on the crush analysis (see Figure 5 for details).

C

onclusion

As it turns out, even when there seems to be limited outward physical evidence to work with, there are still often methods one can apply to solve for closing velocity and impact speed particularly when one is willing to reason beyond oversimplified “rules” limiting otherwise acceptable methods of analysis.

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Two In Three Out Follow-Up from Volume 1 Issue 2

- Greg Russell

ditor’s note: In a previous issue of Collision, we offered the “two in three out” crash test data from the ARC-CSI Crash Conference as a case problem and offered that in a follow up issue, we would continue the discussion of that case problem/crash test. In the ensuing issues, there have been so many fresh and exciting concepts and so many interesting case problems that we all but forgot to run the rest of the discussion. So, in this issue, we’ve reprinted much of the original information plus the remaining discussion in its original format to keep the information all together in one place and offer the reader and opportunity to further explore a solution to the “2 in 3 out” problem.

E

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ackground On May 5th, 2006 during the ARC/CSI conference a crash test was conducted between a 2000 Chevrolet Malibu and a 1991 Geo Metro. Prior to the test the Geo was pre-cut to ensure that during the crash the Geo would separate into two pieces.

B

Even though some may describe this pre cutting as “cheating” as it concerns the separation of the Geo, the fact that the vehicle was pre-cut does nothing to change Newton’s Laws and their application to collision reconstruction. It makes no difference how much, or how little, energy was required to separate a vehicle into two pieces, conservation of momentum still applies (conservation of momentum and energy are not synonymous concepts). Hence, the speeds of the vehicle may still be determined using traditional collision reconstruction methods based on conservation of momentum.

Since this was a staged crash, there are facts about the crash that an investigator who would be working this crash would not have normally known had this been a real world crash. In order to attempt to make this analysis as realistic as possible, it was approached as if this crash had occurred on a public road, with the analysis, when possible, based solely on the scene evidence. The only exception to this approach was the use of the overhead video to estimate the approach angles of the vehicle. Since this collision occurred in an open parking lot as opposed to a roadway, which would have provided evidence as to the pre impact movement of the vehicles and

approach angles, the video was used for this purpose and this purpose only. There were other facts known about the movement of the vehicles post collision that were specifically ignored, since they would not have been known to a investigator investigating the collision had it occurred out on a public road. ost Collision Evidence Despite the nature of this collision and movement of the vehicles when observed on the video, as in many real word collisions, there was little post impact scene evidence that could be used to reconstruct this collision. Considering the violent appearance of the impact, there was little scene evidence associated with this collision. There were no gouge marks in the pavement that could have been used to help identify the area of impact. Nor were there any tire or scrape marks that would have been helpful in determining post impact movement of either portion of the Geo.

P

There was a faint tire scuff produced by the right front wheel of the Malibu. This mark measured approximately 42 feet in length. After the initial 5½ feet of travel this mark veered to the right changing direction by approximately 22°, indicating that there was some type of steering input, whether driver or collision induced, causing the Malibu to change direction after it had separated from the Metro.

An inspection of the Malibu after the collision did establish the right front wheel sustained superficial damage. Nonetheless, the wheel was not locked, was not noticeably out of alignment, and the

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Malibu remained drivable after the collision.

There were two tire scuffs produced in the area of impact. The right rear wheel of the Geo produced the first mark while the right front wheel of the Malibu produced the second mark. ehicle Weights The curb weight for the Malibu was 3039 pounds, while the Geo’s curb weight was 1625 pounds, with approximately 61% resting on the front wheels and 39% on the rear wheels. In the momentum calculations the weight of the Malibu was increased by 225 pounds to account for the driver’s weight and any equipment that had been installed in the vehicle for the crash test. As is the case of most real word cases this weight was an approximation of the additional weight in the vehicle since the driver and other equipment was not physically weighed.

V

The front and rear weights to the Geo were adjusted to account for the location of the split. Since the Geo split behind the center mass, the front and rear portions would not have maintained the 61/39 percent ratio that existed prior to the collision. The rear portion of the Geo was estimated to account for approximately 25% of the vehicle’s pre collision weight while the front section was estimated to account for approximately 75% of the total pre collision weight. These estimations resulted in weight for the rear portion of the vehicle of 406 pounds and 1219 pounds for the front portion.

ost Impact Velocities Post impact velocity calculations are arguably the area within the calculations that requires the most speculation on behalf of the investigator, and is the area where values used in the calculations may be the hardest to defend if confronted.

P

The distance used to determine the post impact velocities for both vehicles was measured from the approximate center mass of each vehicle at the area of maximum engagement to their respective final resting positions, not from the area of impact to final rest. Admittedly, there would be little difference in the final calculations if a post impact distance from impact to final rest, which is the normal practice of many investigators, was used in the calculations, verses the slightly shorter distance from maximum engagement/ separation. Nonetheless, the post impact velocities of the involved vehicles are not defined until maximum engagement has been achieved. If the post impact velocities were defined at impact, then there would be no ∆t in the collision. Instead the vehicles would instantly go from their impact velocity to their respective post impact velocity, which simply is not the case.

Since establishing the area of maximum engagement is somewhat of a subjective process, especially in this case where there was little physical evidence that could be used to identify its location, the distances used in the subsequent calculations could easy vary a short distance. The post impact distance identified for the Malibu was approximately 54.5 feet. The post impact distance for the front portion of the Geo was determined to be approximately 4.5 feet while the distance traveled by the rear portion of the Geo was determined to be approximately 48.5 feet.

The coefficient of friction for the road surface was estimated to be 0.80. This was based on various testing conducted throughout the day at locations throughout the testing site, though no specific test was conducted where this collision occurred. However, even though the coefficient of friction of road surfaces appears to be an area within the field of collision reconstruction that is often extensively debated, its actual value is not overly sensitive in the calculations. If the value of the coefficient of friction was ranged from a 0.75 to 0.85 in this case, it will only cause the impact velocity of Malibu to change approximately

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±1 mph, while the impact speed of the Geo will range approximately ±½ mph.

Ironically, often one of the least debated values used in any individual reconstruction is the percentage of braking that a particular vehicle achieved post collision. Yet this is the one single value that may contain the most error, may be the hardest to defend or support through the use of physical evidence, and may require the investigator to draw on past experiences that cannot be easily articulated. Unless a vehicle has locked all four wheels, which is not common, it cannot be assumed that the vehicle has achieved 100% braking post collision. Nonetheless, this is often the case.

Much too often a case will be reconstructed where there will be no adjustments for the post impact braking of a vehicle in the collision, which will by default assumes the vehicle’s braking percentage is be 100%. Thereby over estimating the both the post impact and impact velocities of the involved vehicles.

Based on the presence of the mark produced by the right front wheel, the fact that there was no damage to the Malibu that would have caused that wheel’s movement to be retarded, and the mark began after the vehicles would have physically separated, it may be concluded that the brakes were being applied post collision.

However, there were no other marks present from the other three wheels, nor was their any specking on the tire treads. Therefore, it may be conclude the percentage of braking was something less

vehicle. The post impact travel distance of the front of the Geo is so short, that virtually any braking percentage may be used while only having a minimal effect on the overall calculations. Since it is known the brakes were not being applied in the Geo after the collision, and the vehicle’s weight was no longer being supported by all four wheels, it may be conclude that post collision braking percentage of the front end of the Geo was low. The reality of the matter is, when the post impact braking percentage for the front portion of the Geo is ranged from 10% to 60% it only produced a 4.7 mph difference in the post impact velocity calculations for that portion of the vehicle. The 4.7 mph range is reduced to under a 3 mph range when dealing with the calculations for the impact velocity of the Malibu. In the calculations to follow the post impact braking percentage for the front half of the Geo was estimated to be 25%.

Due to the fact that there would have been no braking system attached to the rear of the Geo one may surmise that the rear wheels were not locked after the collision (not to overstate the obvious).

Additionally, there were no gouges or tire marks that could be attributed to the post impact travel of the rear portion of the Geo suggesting a lower post impact braking percentage. However, the rear half of the Geo traveled considerably farther than the front half which is not unexpected when looking at the damage to both vehicles. During the course of the collision the Malibu did engage a greater portion of the rear of the Geo than it did the front. Even though the damage to both vehicles indicates that the Malibu drove through the Geo, which would have prevented the rear portion of the Geo and the Malibu from achieving a common post impact velocity, their respective post impact velocities should not have been

drastically different. For the reasons cited the post impact velocity of the rear portion of the Geo was estimated to be 50%.

The post impact velocity for the Malibu based on the values cited is 28 mph while the post impact velocity for the front half of the Geo was 5.2 mph and the rear half was 24.1 mph. pproach and Departure Angles Since this collision did not occur on a roadway where the configuration of the roadway and the traffic patterns could be used in helping to establish the approach angles of the involved vehicles, an image form the overhead video was used. This was the only “known” data that would not have been available to an investigator at an actual crash scene that was used in this analysis.

A

Based on the videos image the approach angle of the Geo was determined to be 145° with the approaching heading of the Malibu set to 0° than 100%. The problem arises as to determining how much less.

If the front wheel had been locked (which it was not) from damage and there were no other marks, then it might not be unreasonable to assume that the only wheel that was braking was the right front wheel and assign a braking percentage of 35% to 45%. Nonetheless, in this case, just as if there had been no marking from any of the wheels, there is evidence that the driver applied the brakes.

Therefore, simply by drawing on past experiences the braking percentage of the Malibu was estimated to be 60%.

Estimating the braking percentage for the Geo becomes even more difficult than estimating the percentage for the Malibu since it separated into two pieces, both of which have to be addressed as a separate entity. The easiest portion to deal with is the front of the www.collisionpublishing.com

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The determining the departure angles of both vehicles becomes more problematic than determining the approach angles or headings. Despite what remains a common misconception, the departure angles are not measured from impact to final rest. As with the post impact speed the departure angle of the involved vehicles is defined at maximum engagement or the point of separation (of the forces). Therefore, the departure angle of the vehicles should be measured from the point of impact through the maximum engagement or separation. If a vehicle were to travel in a straight path after the collision then the departure angle would pass through the center mass of a vehicle as it sat at final rest. However, as in this case, rarely do vehicles travel in a straight line from maximum engagement to final rest.

Obviously, the departure angle of this portion of the Geo is open to debate and discussion. Nevertheless, due to the light weight of the rear portion of the Geo, the departure angle associated with the rear portion of the Geo will have the least amount of influence in the impact velocity calculations. mpact Velocity Calculations Having identified all of the necessary items needed to complete momentum calculations using a traditional approach, the impact velocities of both vehicles may be determined.

Since the Geo separated into two pieces there are two separate approaches that may be taken to determine the impact velocities of both vehicles. The first is to set up the momentum equation to solve for two objects into the system and three objects out. The second method would be to first solve for the post impact velocity and angle of the Geo had it remained intact, then use the traditional momentum equations to solve for the impact velocities.

I

When solving this problem utilizing the first approach, the momentum equation needs to be rewritten to account for both portions of the Geo post collision.

mentum equation.

Beginning with the basic mo-

Rewriting the right side of the equation to take into account both portions of the Geo (sections a and b) The departure angle of the Malibu was determined to be approximately 3.2°. When a line is drawn through the center mass of the Malibu at the point of impact through the area of maximum engagement, it still passes though the center mass of the Malibu when it is positioned along the tire scuff at a point just before it veers to the right.

If the departure angle were incorrectly drawn from the point of impact to the final resting position of the Malibu, the angle would have been misidentified by a significant amount. This error would have had a significant adverse influence on the calculated impact velocity of the Geo. By increasing the post impact heading of the Malibu, while all other values remain the same, the calculated impact speed of the Geo would increase by approximately 25 mph and the Malibu’s by approximately 10 mph.

Since the front portion of the Geo traveled for such a short distance post impact, its departure heading would not have had a chance to vary significantly from maximum engagement to final rest. Therefore, determining its departure heading did not present a significant problem. Whereas, the departure angle of the rear portion of the Geo was not as easy to identify. As previous stated, lacking the necessary roadway evidence, establishing the area of maximum engagement becomes somewhat subjective. Based on the existing evidence the departure angle of the rear portion of the Geo was estimated to be 25.8°.

Converting the angular nature of the collision (approach and departure angles) to a linear relationship by taking the Sine of the various headings At this point the equation that has been developed may be used to solve for any of the involved velocities if all but two of the velocities are known along with the vehicle weights and angles. There is no requirement that the impact velocities have to be solved for, nor is there a requirement that the impact velocity of Vehicle #2 has to be solved first. With some simple algebraic manipulation of the above equation any of the velocities may be solved for. Since 1 becomes before 2, it seems logical to solve for the impact velocity for Vehicle #1 (Malibu) before solving for Vehicle #2’s (Geo).

To solve for Vehicle #1’s impact velocity Subtract w2×v2×sin(Θ2) from both sides

Divide both sides by w1×sin(Θ1) to isolate v1

At this point an equation has been developed to solve for the impact velocity for Vehicle #1 that is not functional since both the left and right and left side of the equation contains an unknown value (v1 and v2) and since Vehicle #1’s approach headings set to zero, the equation would be dividing by zero, further rendering it invalid. However, if the angles were adjusted so that one of the other angles were set to zero, the terms tied to that value through multiplication would drop from the equation (the Sine of 0 = 0 and 0 × any value = 0).

Adjusting any one of the vehicles angles does not require the angles to be remeasured. For instance to drop Vehicle #2’s impact velocity to 0 in the above equation, Vehicle #2’s approach heading needs to be set to zero. To do this one simply has to subtract its approach heading from itself, 145°-145°=0°. However, what is done to one angle has to be done to all of the other angles, including the approach heading for Vehicle #1. Therefore, in this case the approach angle of Vehicle #2, 145° has to be subtracted from the other three

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angles. (Do not convert negative results to positive values. 0°-145° = negative 145°) Subtracting the approach angle of v2 from all angles to solve for v1 Since w2×v2×sin(Θ2-Θ2) = 0, w2×v2×sin(Θ2-Θ2) drops from the equation, leaving:

If the post impact momentum vectors of the Geo are drawn to scale end-to-end, the summation vector will represent the Geo’s post impact momentum vector had it remained intact.

As illustrated in the two below diagrams, it makes no difference which post impact vector is drawn first. Both the length and the angle of the resulting vector or the third side of the triangle remains the same. To convert the total post impact momentum of the Geo into a velocity, the total post impact momentum simply needs to be divided by the total weight of the Geo. This yields a post impact velocity for the Geo, as a single unit, of 14.5 fps or 9.9 mph.

Solving for the impact velocity of the Malibu (Vehicle #1)

The same process may now be used to solve for the impact velocity of the Geo, Vehicle #2 without making any adjustment to the angles.

To solve for Vehicle #2’s impact velocity Starting with:

Since w1×v1×sin(Θ1) =0, it drops from the equation, leaving:

Divide both sides by w2×sin(Θ2) to isolate v2:

Solving for the Geo’s impact velocity

The post impact velocity of the Geo may be determined mathematically base on the same values that were used to complete the vector diagram. Since two sides of the triangle are known as well as the angle that joins them (which may be calculated from the known data), the third side of the triangle, total post impact momentum may be determined using the Law of Cosines.

The angle that joins the two post impact vectors may be determined by subtracting the two departure angles from each other, then taking the absolute value of the result and subtracting it from 180°.

A second approach to solving for the impact velocities would be to first solve for the post impact velocity and departure heading the Geo would have had, had it not separated into two sections.

Though this is not necessary to determine the impact velocities, as just shown, it is necessary to determine the ∆v and pdof. This set of calculations may be completed through the use of the law of Cosines.

The below diagrams help illustrate this process. If all of the post impact vectors for the Geo are rotated until one of the post impact vectors has an angle of zero when measured off the reference line, in this case a horizontal line or red vector in the bottom diagram.

The angle formed between the reference line and the other post impact vector, blue vector, will represent the absolute value of the two departure headings subtracted from each other. |13.4-25.8| = 12.4, |25.8-13.4|=12.4. When this result is subtracted from 180, it will equal the angle that joins the two vectors. 180-12.4=167.6.

The departure heading of each portion of the Geo and their respective departure vectors represent two sides of a triangle, whose third side is made up by the vector that represents the post impact heading and post impact momentum of the Geo. It is important to note that the sides of triangle being discussed represents units of momentum and not velocity. Since the two portions of the Geo have different weights, the vectors have to be addressed in units of momentum.

The convert the post impact velocities for each portion of the Geo to momentum, each post impact velocity has to be multiplied by its respective weight.

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When solving for side a of a triangle the Law of Cosines is written as:

In this case side b represents the one of the post impact vectors while side c represents the other and angle A is the angle that joins the two lines.

When our known values are inserted into the Law of Cosines equation, the equation becomes.

It is important to note that at this point due to the weight differences of the two portions of the Geo the equation still has to be written to work in units of momentum. In order to have the above equation solve for velocity, the equation has to be rewritten to account for the differing weights. Once this has been done the equation may be expressed as:

The angle that is identified by the previous process will represent one of the angles of the vector diagram and not the departure angle.

In order to determine the departure angle the result form the previous set of calculations needs to be added to the departure heading for the first portion of the Geo, 7.5 + 13.4 = 20.9°.

If the post impact vectors are drown in the opposite order, as in the below diagram, the previous process will remain the same. However, the various sides of the triangle will represent different sides of the triangle. Therefore, the previous equation will produce a different result. In this instance the value returned by the equation needs to be subtracted from the departure heading of the second portion of the Geo to arrive at its common post impact heading.

25.8 – 4.9 = 20.9.

The above equation yields the same result that was produced by the vector analysis, as it should.

With the total post impact velocity of the Geo identified, the triangular relationship of its post impact vectors may be returned to along with the Law of Cosines to identify its single unit post impact heading.

Starting with a variation of the Law of Cosines written to solve for side b.

Add 2×a×c×cos(B) to each side:

Subtract b² from both sides:

Once the Geo’s single unit post impact heading and velocity have been identified, the impact velocities of both vehicles may be solved using the traditional 360°momentum equations without any adjustments.

eo Departure angle The single unit departure angle for the Geo, along with its total post impact velocity is necessary in order to determine the ∆v of the vehicle along with the pdof. Aside from the process just detailed this angle may be determined using the conservation of linear momentum equation. However, in this set of calculations three of the four involved velocities, to include the total post impact velocity of the Geo have to be known. Therefore, this analysis is not useful in determining the impact velocity of the involved vehicles. Nonetheless, unlike the previous analysis, this one will only produce a single result.

G

Returning to the basic form of the conservation of momentum equation.

Divide both sides by 2×a×c:

Take the inverse cosine of both sided to solve for B:

Since the equation will be derived to solve for the post impact heading of the Geo as a single unit, and post impact velocity of the Geo has been identified, v’2 does not have to be split into two components.

Taking the sine of the approach and departure angles to transform the angular relationship into a linear relationship

Insert known values and solve.

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the Malibu’s post impact velocity, its departure angle and estimated pdof may be used to determine its impact speed independently of the movement of the Geo. Therefore, if the impact velocity of the Geo is not a concern in the investigation, no information about the Geo has to be known. Hence, the fact it spilt into two separate pieces never has to be addresses.

Subtract , w1×v’1×sin(Θ’1) from each side:

Divide both sides by w2×v’2 and rearrange:

Take the arc sine of inverse sine of both sides to solve for Θ’2:

elta v and pdof Calculations The ∆v equation is another variation of the Law Cosines.

When the vector relationship between the impact momentum, post impact momentum and the change in momentum is examined, once again a triangle is established where two sides and the adjoining angles are known. The impact and post impact momentum/velocities and the change in direction, which is the difference between the vehicle’s approach and departure headings. Using these values, the change in velocity may be determined for each of the involved vehicles.

D

When using this approach the post impact velocities and the departure angle for the Malibu would be determined using traditional methods. Therefore, the values used previously will be used in this analysis.

The only data needed to complete the impact velocity calculations for the Malibu is its pdof. However, in a case such as this, the pdof estimation will not be overly sensitive to the calculations. When looking at the post impact travel of the Malibu in relationship to travel of the Geo, it can be seen that the Malibu did not deviate far from its approach heading and there was little post impact travel.

These two factors alone will dictate that the pdof for the Malibu is going to be relatively shallow.

Once the change in velocity has been determined, the length of all three sides of the triangle are known. Therefore, the Law of Cosines may again be used to determine any of the three related angles. In this case the principle direction of force.

orking with velocities other than post impact velocities Often in many real world collisions and in this case, determining a reliable post impact velocity for one or both of the vehicles may be difficult, leading many investigators to believe that the velocities of the involved vehicles may not be determined. This is simply not the case. Often, there are alternative methods to determine velocities utilizing conservation without relying on post impact velocities of both the vehicles.

W

When looking at the front end of the Malibu there is very little noticeable side shift of the front end as a result of the collision.

Further suggesting the pdof of the Malibu is shallow.

In the analysis to follow the pdof of the Malibu was estimated to be -10° (the negative indicates that the came from the left)

To make the crash test more closely relate to a real world crash, a roadway has been superposed over the scale diagram of the actual crash data. Once this has been completed it may be envisioned how this collision could have occurred in the real world.

The ∆v that is recorded by the SDM module represents the ∆v the vehicle experienced along its X axis, from front to rear. Unless the collision involved a true head-on collision, the ∆v reported by the SDM will not reflect the ∆v that the vehicle experienced along the pdof, the actual ∆v the vehicle underwent as a result of the collision forces. Therefore, before the SDM ∆v can be used properly in any analysis it has to be adjusted to reflect the ∆v along the pdof.

This Malibu was equipped with a SDM which did record data as a result of this collision. The ∆v recorded by the SDM along with

To adjust the SDM ∆v to reflect the ∆v along the pdof relies on a basic trigonometry function. The ∆v along the X axis of the vehicle

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along with the ∆v along the pdof will form a right triangle where pdof angle will form the reference angle. Hence, the SDM ∆v will represent the adjacent side of the triangle while the ∆v along the pdof will represent the hypotenuse of the right triangle. The adjusted ∆v will then be the SDM ∆v divided by the cosine of the absolute value of the pdof.

as a result of the collision. Nor is there any damage to the left side of the front of the vehicle, which is clearly visible in the photograph on the previous page. Yet the collision forces do come from the Malibu’s left, the direction the Geo was approaching from.

Since the podf acts on the centroid of damage, there is no requirement that it pass through the center of the vehicle or cross either the vehicle’s X or Y axis.

The diagram to the right illustrates the approximately location the pdof was applied to the Malibu as a result of the collision with the Geo. It may be seen that the pdof does come from the Malibu’s left, though there is no contact to the left side of the Malibu.

In this case the 6.33 mph ∆v recorded by the SDM represents a ∆v of 6.4 mph along the pdof, not a significant difference.

The chart to the right illustrates that in this collision, which may not be the case in all collisions, the accuracy of the pdof estimation will not be critical to the subsequent calculations. Based on the nature of this collision and the damage sustained by Malibu, there is no argument that the Malibu’s pdof is shallow, or numerically small. Whether the actual pdof is 0°, -5°, -10°, -15° or -20° is not particularly important in the analysis since it only produce a 1 mph change in the calculated ∆v. This 20° variance only produces a 0.1 mph difference in the impact velocity calculations of the Malibu.

Therefore, in this case a reasonable estimation of the pdof, which is not difficult to perform, is all that is required.

Once the ∆v along the pdof has been determined the Law of Cosines may once again be used to determine the impact velocity of the Malibu based on its departure angle, post impact velocity and ∆v. The below vector diagram (which does not represent the Malibu’s vector diagram, which would not reproduce well due to the angles involved) again illustrates the relationship between the three velocities, impact, post impact and ∆v, and the various angles. Having determined the post impact velocity, ∆v along with the pdof and change in direction, it may be seen that there is only one possible way the third side of the triangle, impact velocity, may be formed.

The Law of Cosines is dependant on two sides of a triangle and the angle that adjoins them. Therefore, in order to calculate the impact velocity, the post impact velocity and the ∆v have to be used in the calculations. This in turn requires that angle C in the above triangle has to be used in the calculations which has not been identified.

However, since the sum of all three angles of any triangle equals 180°, if the value of two of the three angles are known, pdof and change in direction, the third angle is also known since it will equal 180°- the sum of the other two angles.

Law of Cosines solving for side a

Inserting known values and solving:

A word of caution when estimating the pdof. The pdof represents the direction the collision force, not where the damage is on the vehicle. In this case there is no damage to the left side of the Malibu 100 Collision

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The results of the calculations produce an impact velocity for the Malibu of 33 mph, which is about 4 mph lower that the results of the momentum calculations.

Not only is the weight of the Malibu not required in the calculations that have just been performed, but impact velocity of the

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Malibu has been conducted without any information from the Geo. The movement, or the fact that the Geo separated into two pieces has no relevancy to these calculations. If there were a real collision where the speed of the Geo or its pre impact movement was not an issue (such as if it stopped at a stop sing on not) then this analysis may be conducted without even knowing what type of vehicle the Malibu struck.

As the below chart illustrates, neither the departure angle of the Malibu or the estimated pdof are particular sensitive to the resulting calculations. As eluded to earlier, the single variable that produces the greatest deviation in the results if the percentage of post impact braking that has been attributed to the Malibu after the collision. In this case, when ranging the post impact braking percentage from 40% to 80% will produce an approximate 9 mph range in the impact velocity calculations. Whether or not this presents a problem to the investigator will depend largely on the circumstances around the collision. pply the Malibu’s data to the Geo Often overlooked is the fact that the forces involved in a collision are not only equal but opposite. This means that if you know the pdof of one of the involved vehicles along with the relative approach angles, then you know the pdof the second vehicle (It has to be opposite the first). However since each vehicle’s pdof is measured relative to the direction the center mass was traveling on impact, 180° cannot be added to one vehicle’s pdof to arrive at the other vehicle’s pdof. In order to determine the pdof of one vehicle based on the pdof of the other we have to again return to the triangular relationship that exists between the vehicles.

A

In the above vector diagram (which does not represent this collision) the magenta line is an extension of the impulse vector for the second vehicle that has been extended until it intersects with the impact vector of the first vehicle. This forms a triangle that is comprised by the impact vector of the second vehicle, a portion of the impact vector of the first vehicle and the impulse vector of the second vehicle. Since the impulse vectors are equal and opposite, the angle that is formed by the extension of Vehicle #2’s impulse vector and Vehicle #1’s impact vector is the same as Vehicle #1’s pdof (green shaded angles). This represents one of the three angles of the triangle that has been defined. The second angle of the same triangle is formed by the impact vectors of both vehicles. Therefore, this angle will represent Vehicle #2’s approach heading (assuming Vehicle #1’s is set to zero). Now that two of the three angles of the triangle have been defined, the third angle, the pdof of Vehicle #2, will simply be the sum of the other two angles subtracted from 180°.

Based on the estimated pdof of the Malibu of -10° in the previous calculations and the Geo’s approach angle of 145°, the Geo’s pdof would equal 25°.

Just as the pdof of one vehicle may be determined based on the pdof of the second, the same holds true for the vehicle’s change in velocity. The only additional data required to determine ∆v of one vehicle based on the ∆v of the other is the vehicle weights.

The impulse or change in momentum (∆p) experienced by one vehicle is opposite the ∆p experienced by the other. (∆p should not be confused with ∆v, which in most cases will not be equal for both vehicles.) The ∆p is a function of the vehicles’ acceleration (velocity) and its mass. The mass of the vehicle may be divided by gravity throughout the momentum equation, permitting the use of weight (pounds) instead of mass (slugs). Therefore, the impulse or ∆p may be defined as: ∆p = the vehicle’s change in velocity multiplied by its weight, ∆p = ∆v × w. Since the impulses have to be equal and opposite (there are no exceptions), if the ∆p is known for one vehicle, by default it is known for the second. Therefore in order to determine the change in velocity for either vehicle involved in the

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collision, all that needs to be done is the ∆p needs to be divided by that vehicle’s weight.

Post impact velocity of the Geo

Multiplying the pdof adjusted ∆v from the Malibu to determine the ∆p then dividing the result to determine the Geo’s ∆v.

Departure angle of the Geo Having established both the ∆v for the Geo, based off the Malibu’s SDM data, along with the pdof, which was also based on the Malibu, one side and one angle of the triangle comprised of the Geo’s impact velocity, post impact velocity and ∆v vectors has been defined. The only data that is required to determine what the remaining values are is one of the reaming unknowns, impact velocity, post impact velocity or departure angle.

At this point the velocity calculations of the Geo, though based on the Malibu’s SDM data, remain independent of the Malibu’s SDM based velocity calculations. Therefore, the Geo’s impact velocity, post impact velocity, vehicle weights and approach and departure angles may be used to determine Malibu’s impact and post impact velocity using conservation of momentum.

Starting with the basic momentum equation when taking into account the approach and departure angles.

Rewriting the equation to solve for the Malibu’s post impact velocity

Rewriting the equation to solve for the Malibu’s impact velocity In this scenario presented in the diagram to the right, where the Geo is attempting to make a left turn, which would be consistent with the nature of the this collision. It would not be difficult to arrive at a reasonable impact speed based of sampling turning traffic at the same location, the radius of the curve, or statements from the driver of witnesses. For instance, had the driver of the Geo been stopped prior to attempting their left turn, then based on the acceleration capabilities of the Geo as well as the radius of the left turn, 15 mph would not be an unreasonable estimation as to the impact velocity of the Geo. Using the 15 mph impact velocity calculation along with the ∆v and pdof, the post impact velocity of the Geo may be determined along with its departure angle. When looking at the partially completed vector diagram consisting of the known values, ∆v, impact velocity and pdof, it can easily be seen that there is only one possible vector that may be drawn to complete the triangle. Once again this side of the triangle, post impact velocity and the departure angle of the Geo may be determined utilizing the Law of Cosines or one of its derivatives.

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What is important to note about this analysis is it relies on very little scene data. The approach angle of both vehicles and the departure angle of the Malibu were derived from the scene data while the departure angle of the Geo was calculated. This eliminates any issues concerning the departure paths of the two separate pieces of the Geo. The impact velocity of the Geo was estimated. However, often in real world collision there will be sufficient information where this velocity may be reasonably determined. The remaining three velocities, to include both the post impact velocity of the Geo and the Malibu, as well as the impact velocity of the Malibu are derived from the SDM data. This eliminates the need to know what the coefficient of friction of roadway was, either vehicle’s post impact braking percentage, whether a vehicle remained intact after the collision or split into multiple pieces, or how far either vehicle traveled after the impact. Aside from the departure angle of the Malibu, absolutely no post collision data entered into this analysis.

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New CDR Release!

and

BOSCH CDR 3.1 Software & Hardware CDR System Software Version 3.1

GM

This new software release enables the retrieval of Event Data Recorder (EDR) information from select 2004 – 2009 Chrysler vehicle airbag control modules (ACM), select 2001 – 2009 Ford ACMs, 2003 – 2007 Ford powertrain control modules (PCM) equipped with Electronic Throttle Control, 1994 – 2009 GM ACMs and 2007 - 2009 GM Roll-over Sensors (ROS).

Details on the specific vehicle coverage is found in the CDR help file.

Windows Vista Compatibility

New Vehicle Coverage for 3.1 Chrysler • •

2009 Carry over of currently supported 2008 vehicles 2008 – 2009 Chrysler 300 and Town & Country

Dodge • • • •

Jeep • •

Ford • • • • •

2009 Carry over of currently supported 2008 vehicles 2008 – 2009 Dodge Charger, Challenger, Dakota, and Grand Caravan 2008 Dodge Magnum 2009 Dodge Journey 2009 Carry over of currently supported 2008 vehicles 2008 – 2009 Jeep Grand Cherokee and Commander 2007 - 2008 Edge 2008 Focus 2009 F150 2009 Escape (hybrid/gas) 2009 Econoline

Lincoln •

2007 - 2008 MKX 104 Collision

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•

2009 GM vehicle coverage includes ACM support or most cars and light & medium duty trucks in addition to 2007 - 2009 Roll-Over Sensor support for applicable trucks and SUVs.

Version 3.1 is now compatible with Windows Vista operating system while maintaining compatibility with Windows 2000 and Windows XP (Pro and Home for PCs). Windows 98 and NT operating systems are no longer supported.

Version 3.1 Software and CDR Interface Module Compatibility This software version is compatible only with the blue and green metal CDR interface modules marked with a CAN plus logo on its label. Other versions of the CDR module (black plastic or blue units without the CAN plus logo) are not supported and can cause the software not to function.

New CDR Hardware Available this Release To take full advantage of the new CDR software, you need to make sure you have the latest CDR hardware.

The hardware kits listed below have been added for CDR v3.1.

CDR v3.1 Upgrade Kit: MSRP $699 P/N: F-00K-108-426 The CDR v3.1 Hardware Upgrade Kit includes all newly added cables and adapters available for version 3.1 CDR software release. Buy this kit if you purchased your CDR System prior to this release.

This new kit includes four new cables (3 Ford and 1 Chrysler) and an ACM Adaptor needed to download www.collisionpublishing.com

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EDR data from select Ford and Chrysler airbag control modules (ACM) added in this release.

NOTE: Some 1994-1995 supported GM vehicles require the 02003090 ALDL cable for DLC downloads which is not included in this kit; Depending on the condition of the vehicle, crash data may not be accessible thru the vehicle’s DLC connector; 2008 – 2009 Chrysler/Dodge mini-van & 2009 Dodge Journey currently do not support crash data downloads through the DLC connector, only direct connection to the ACM using the F-00K-108-385 is supported on these vehicles.

Manufacturer Specific CDR ACM Cable Kits •

Chrysler ACM Cable Kit: MSRP $499 P/N: F-00K-108-390 Includes four ACM cables which are required to retrieve crash data when connecting directly to ACMs on supported Chrysler vehicles.

This new CDR hardware kit supersedes the CDR version 3.0 hardware kit (P/N: F-00K-108-329). The kit includes all direct connection ACM cables and CDR System components that are needed to support CDR software version 3.1 (The Ford PCM Kit is not included and sold separately). It includes all the new components in the CDR v3.1 Hardware Upgrade Kit.

•

Ford ACM Cable Kit: MSRP $879 P/N: F-00K-108--391 Includes six ACM cables and an ACM Adapter which are required to retrieve crash data when connecting directly to ACMs on supported Ford vehicles. (Ford PCM Kit is not included and is sold separately)

CDR System Starter Kit: MSRP $1356 P/N: F-00K-108-388

•

GM ACM Cable Kit: MSRP $1100 P/N: F-00K-108-392 Includes nine ACM cables and a Power Booster Adapter which are required to retrieve crash data when connecting directly to ACMs on supported GM vehicles.

•

Ford PCM Interface Kit (not new for v3.1) P/N: F-00K-108-277: MSRP $556 Includes three PCM cables and a PCM Adapter which are needed for retrieving crash data directly from PCMs on supported Ford vehicles.

Premium CDR System Kit: MSRP $3799 P/N: F-00K-108-389

The CDR System Starter Kit is a low cost, entry level kit which includes only components needed to retrieve crash data from ACMs through the DLC connector of 1994 and newer vehicles supported in CDR software version 3.1.

These newly added kits allow CDR users to purchase additional CDR cables specific to the vehicle manufacturer without having to buy an entire CDR kit or order cables individually.

Contact CRASH DATA GROUP for ordering and special pricing information.

Phone: 800-280-7940 Web: www.cdr-system.com www.collisionpublishing.com

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106 Collision

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CSV Pro 2008 Features • Easily import CDR System comma separated values (CSV) files into Excel to allow for end user selectable graphing and display options • Automates the often complex graphing and table display tasks in Excel when importing CSV files • Easily converts the many hundreds of individual cells found in a CSV file in Excel to easy to read graphs and tables • Allows analysts to evaluate CSV files without Restraint Deployment Signals to find and format the output start and stop points for a more complete analysis • Retains original exported content while allowing for end user formatting

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ollision C Fall 2008

Volume 3 Issue 2

In this issue

“Multiple Case Studies” Plus . . .

Vehicle Speed Sensor Calibration Basic Integral Calculus

for Crash Reconstruction

Bus Braking Tests: also Close-Range Photogrammetry & Forensic Photography

With and Without ABS

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