chapter 4: design considerations and guidelines Design Considerations
Travel Lane Widths
There are several overarching street design elements inherent to creating Complete Streets. These considerations, such as block length and street width, contribute to the overall safety and desirability of bicycling and walking. The following section presents an overview of these considerations as well as guidelines where appropriate.
A common misconception is that wider motor vehicle lane widths provide more safety and/or automobile capacity. Research has shown that reducing travel lane widths from 12 feet to 11 or even 10 feet has little negative impact on motor vehicle traffic—safety, movement and flow, and in fact there is a demonstrable postive impact on safety with narrower lanes (Noland 2003).
A G UIDE TO COMPLETE STREET S
Street Widths When considering street widths, there are three factors to be considered: 1) lane width; 2) total number of lanes and 3) total street width. Streets are often designed to wider standards than necessary, which negatively influences the safety of users by facilitating faster vehicle speeds and increasing crossing distances at intersections. Streets over 60 feet wide can present barriers for pedestrians and facilitate higher vehicle speeds while discouraging crossings for transit connections (Institute of Transportation Engineers 2010). Research has shown that narower streets calm traffic and improve safety and crash risk for all users. One study found the most significant relationship between injury crashes and street design to be width and curvature (Swift 2003). According to this study, the safest residential street width (curb to curb) was 24 feet. Wider streets not only result in a higher average 85th percentile speed, but also present longer crossing distances for pedestrians (Daisa and Peers 1997); (Ridgway 1997). In addition to slowing traffic and creating a pedestrian-friendly environment, narrower streets reduces the environmental impacts associated with pavement (urban heat island effect, runoff). Example Residential Street Width Standards (Kreck 1998) City
Width
Portland, Or
18’-24’ depending on parking
Madison, Wi
27-28’, parking on both sides
Novato, Ca
20-28’ with parking on both sides
AASHTO has identified benefits to narrower travel lanes (10 to11 feet) on lower-speed streets, such as reduced pedestrian crossing distances. In addition, by reducing travel lanes (for instance from 12 feet to 10 feet), enough space can be provided to add bicycle lanes and/or buffers between sidewalks and the travel-way. Reducing lane widths is also a good traffic calming strategy on roads where excessive 85th percentile speeds are an issue. The Institute of Transportation Engineers (ITE) recommends selecting lane widths based on the target speed, design vehicle, available right-of-way and the width of adjacent bicycle and parking lanes. The ITE suggests a range of lane widths from 10 to 12 feet on arterials and 10 to 11 feet on collectors. On arterials with target speeds below 30 mph, 10 to 11 feet widths are appropriate. On collectors with a target speed below 30 mph, a 10-foot lane width may be appropriate (Institute of Transportation Engineers 2010). Block Length A longer average block length not only increases the 85th percentile speed of streets, but also negatively affects the pedestrian and bicycle experience. Longer blocks increase the likelihood that a pedestrian will cross between intersections to access destinations, whereas shorter blocks provide increased connectivity and route choices for pedestrians and bicyclists. In a study out of San Antonio, Texas, streets with blocks exceeding 600 feet in length had 85th percentile speeds that exceeded the legal speed limit (Sacramento Transportation and Air Quality 2005). ITE (2010) recommends spacing pedestrian facilities so block lengths in less dense areas (suburban or general urban) do not exceed 600 feet (preferably
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