The report Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions was released on Tuesday by the National Academy of Sciences. Congress commissioned the study four years ago, and a wide range of transportation experts have been putting this together. The Transportation Research Board is a pretty noteworthy bunch, with representation from FHWA, FTA, many major universities, many state DOTs, AASHTO, and even a Walmart executive. The Chair of the committee tasked with writing the report is José Gomez-Ibáñez, Professor of Urban Planning and Public Policy at Harvard University.
Measuring the effect of the built environment on Vehicle Miles Traveled (VMT) is tricky, because socioeconomic factors and self-selection tend to skew the results. Do the surroundings cause less driving or do residents of compact, mixed-use neighborhoods just happen to have disproportionately less automobile access? Or perhaps people who prefer not to drive as much choose to live in such places. Even when controlling for these variables, the findings do confirm what smart growth advocates have claimed (although with perhaps less bluster),
“Both logic and empirical evidence suggest that developing more compactly, that is, at higher population and employment densities, lower VMT.”But there is a caveat, and this is something that Kaid Benfield has been stressing lately. Density alone is not enough. We only focus on it, because it happens to be so easily quantifiable.
“Doubling residential density alone, without also increasing other variables, such as the amount of mixed uses and the quality and accessibility of transit, will not bring about a significant change in travel.”So what are the other variables? The report lists them as the five D’s:
- "Density: Population and employment by geographic unit (e.g., per square mile, per developed acre).
- Diversity: Mix of land uses, typically residential and commercial development, and the degree to which they are balanced in an area (e.g., jobs-housing balance).
- Design: Neighborhood layout and street characteristics, particularly connectivity, presence of sidewalks and other design features (e.g., shade, scenery, presence of attractive homes and stores) that enhance the pedestrian and bicycle friendliness of an area.
- Destination accessibility: Ease or convenience of trip destinations from point of origin, often measured at the zonal level in terms of distance from the central business district or other major centers.
- Distance to transit: Ease of access to transit from home or work (e.g., bus or rail stop within 1/4–1/2 mi of trip origin)."
In terms of actual case studies, Virginia’s own transit-oriented development along the Arlington corridor was one of two success stories featured in the report.
“Like Portland, Arlington County demonstrates what can be done through a combination of land use plans and transit investment to promote development and at the same time reduce automobile travel. The county’s success can be attributed to leadership and early recognition of development potential; good planning and design, including rezoning of land adjacent to metrorail stations to allow high-density development; a healthy economic base; and above all, the foresight to take advantage of massive investment in a new regional transit system to channel development.”Is a shift to higher densities actually feasible in the marketplace of many metro areas? The study echos what groups like the National Association of Realtors have been saying recently: the consumer demand for compact, mixed-use developments is outpacing the current supply. There are, however, usually regulatory impediments facing developers who wish to tap this market.
“A population that is aging and includes more immigrants and young adults with urban preferences is likely to be more inclined to live in more compact developments, own fewer automobiles, drive less, and use alternative modes of transportation. Should they occur, sustained higher energy prices would reinforce these trends.”So what’s the bottom line for those who need a quick answer:
“Recommendation 1: Policies that support more compact, mixed-use development and reinforce its ability to reduce VMT, energy use, and CO2 emissions should be encouraged.”(The other recommendation is: keep studying this)
See also last month's Moving Cooler report, led by Urban Land Institute, for a similar analysis with some more specific policy proposals.