by Gary Taylor
The key policy recommendation of a report recently released by the National Academy is that “policies that support more compact, mixed-use development and reinforce its ability to reduce vehicle miles traveled, energy use, and CO2 emissions should be encouraged.” The study, Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO2 Emissions, was requested by Congress and funded by the U.S. Department of Energy to examine the relationship between land development patterns and motor vehicle travel in the United States. Specifically the committee charged with conducting the study looked at whether petroleum use (and therefore CO2 emissions) could be reduced by more compact, mixed-use development. Currently, 80 percent of Americans live in metropolitan areas, but population and employment are increasingly decentralized. Compact, mixed-use development — individuals living in denser environments with jobs and shopping close by — is consistently promoted as a way to reduce the number of vehicle miles traveled by shortening trip lengths, and by making walking, biking, and public transit more viable alternatives to driving.
Key findings of the study:
- Significant increases in density would result in modest short-term reductions in personal travel, energy use, and CO2 emissions. However, these reductions would grow over time. In order to quantify the potential effects of compact development, the committee developed illustrative scenarios, looking forward to 2030 and 2050. If 75 percent of new and replacement housing units in the U.S. were developed at twice the density of current new development, and individuals drove 25 percent less — the committee’s upper-bound scenario — personal travel, fuel use, and CO2 emissions would be reduced by 7 percent to 8 percent by 2030, when compared to a base estimate using current trends, and by 8 percent to 11 percent by 2050. If only 25 percent of housing units were developed more compactly, and residents drove 12 percent less, then personal travel, fuel use, and CO2 emissions would be reduced by approximately 1 percent by 2030, and by 1.3 percent to 1.7 percent by 2050. If in this lower-bound scenario residents drove only 5 percent less, then personal travel, fuel use, and CO2 emissions would be reduced by less than 1 percent by 2050.
The most reliable research studies estimate that doubling residential density in a metropolitan area might lower vehicle miles traveled between 5 percent and 12 percent, and perhaps by as much as 25 percent if higher density were paired with more concentrated employment and commercial locations, and combined with improvements to public transit and other demand management measures.
The committee disagreed about the feasibility of achieving the target density in the upper-bound scenario (doubling the density of 75 percent of new development by 2050). Some members of the committee thought that these higher densities would be reached due to macroeconomic trends — higher energy prices and carbon taxes — in combination with growing public support for infill development, investments in transit, and higher densities along transit rail corridors. Other members thought that the high-density scenario would require such a significant change from current low-density development patterns, land-use policies, and public preferences that it is unrealistic without dramatic changes in all three. In the near term, the primary opportunities to increase density are in areas already experiencing such changes: the inner-ring suburbs and areas close to public transit or along major highway corridors. Over the long term, adopting compact development would likely require changes in housing preferences and a greater political consensus – and willingness of homeowners in existing neighborhoods – to incorporate compact development into local zoning regulations. Public infrastructure investments, such as public transit, and market-based strategies like congestion pricing and higher parking fees, could also be used to steer communities toward compact, mixed-use development.
The full report may be accessed here
A four page summary of the report is available here.