6 metropolitan areas have longer commutes and travel times than those living in more central locations, regardless of the density at the place of residence (Barnes & Davis, 2001). The further people live from destinations, other modes of transportation generally becomes less convenient and accessible. As suggested throughout this paper, residential density by itself may not produce the desired benefits in terms of reducing VMT and increasing bicycling, walking and transit use. However when coupled with other variables, often referred to as the “4D Indices” – density, diversity, design and destinations – the percentage change in vehicle miles or trips has the potential to be much greater. One example shows that by doubling residential density, per-household vehicle trips would reduce by 4 percent, whereas doubling all four of the variables (residential or job density, jobs/housing diversity, neighborhood design, destinations), vehicle trips would reduce by 15 percent (Smart Growth Planning). Other calculations have been made evaluating the influence of land use factors on vehicle travel, as illustrated in the Table 1 below. These calculations suggest that neighborhood design factors can reduce per capita automobile travel between 10 and 20 percent, whereas the proximity of the neighborhood to urban centers can reduce travel by 20 to 40 percent. One example from Davis, California, shows that people who live in a central location typically drive 20 to 40 percent less and walk, bicycle and use public transit two to four times more than they would if in an urban-fringe location (Litman, 2011). The US EPA has incorporated these calculations into their Smart Growth Index Model as a metric for predicting the impact of land use strategies on transportation outcomes (Criterion Planners/Engineers Inc., 2002). Table 1: Typical Elasticities of Travel With Respect to the Built Environment (Ewing & Cervero, Travel and the Built Environment: A Synthesis, 2002) Factor
Description
Trips
VMT
Local Density
Residents and employees divided by land area
-5%
-5%
Local Diversity (Mix)
Jobs/residential population
-3%
-5%
Local Design
Sidewalk completeness/route directness and street network density
-5%
-3%
Regional Accessibility
Distance to other activity centers in the region
--
-20%
To summarize the literature, the following general conclusions have been drawn through Todd Litman’s synthesis of The Effects of Land Use Patterns on Travel Behavior: Per capita automobile travel tends to decline: •
With increasing population and employment density
•
With increased land use mix
•
In areas with connected street networks
•
In areas with bicycle and pedestrian friendly streets and traffic calmed streets
•
With the presence of a strong transit system and good pedestrian access
•
In areas of larger and higher-density commercial centers
Literature Conclusions The relevance of the literature to the premise of this paper is to demonstrate that development and land use patterns have a significant influence on travel behavior. While residential density is an important factor, in order to reduce VMT and SOV trips to the degree supportive of environmental and smart growth objectives, density should be located in areas that provide access to major employment centers, support efficient transit service, sustain mixed land uses and exhibit connected street networks.