The Transportation Research Board (TRB), a division of the National Research Council, today released the report "Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use and CO2 Emissions." Requested by Congress and funded by the U.S. Department of Energy, the report examines the relationship between land use development patterns and vehicle miles traveled (VMT) in the U.S. to assess whether petroleum use and greenhouse gas emissions (GHG) could be reduced by changes to population and employment density.
The Findings
The report found that increasing population and employment density in metropolitan areas could reduce vehicle travel, energy use and CO2 emsisions from between 1 percent and 11 percent in 2050, compared to a base case for household vehicle usage.
Even though 80 percent of Americans live in metropolitan areas, according to the report, population and employment are increasingly decentralized. Doubling residential densitiy in a metropolitan area might lower household driving between 5 and 12 percent, the report found. Pairing higher density with strategies to reduce automobile travel such as improved public transit and concentrated employment and commercial locations could lower household driving by as much as 25 percent.
"In the near term, the biggest opportunities for more compact, mixed-use development are likely to lie in new housing construction and replacement units in areas already experiencing density increases, such as the inner suburbs and developments near transit stops and along major highway corridors or interchanges," the executive summary of the report stated.
But the committee disagreed about the feasibility of achieving the target density in the upper-bound scenario presented in the report. Some committee members thought that 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 departure from current low-density development patterns, land-use policies and public preferences that it is unrealistic without a strong state or regional role in growth management.
Obstacles
A number of obstacles stand in the way of widespread compact development in the U.S., according to the report. To achieve more compact development, states and regional entities would need to take a major role in managing local growth, which is currently controlled by local governments. Local zoning regulations would be a large obstacle, and compact-development initiatives could meet resistance from existing homeowners and politicians, the report said. Their legitimate concerns about congestion, local taxes or home values may be at odds with regional and national goals, such as housing affordability and climate change.
Over the long term, the report said adopting compact development would likely require changes in housing preferences and a greater political consensus in support of stronger state and regional control of land use. Public infrastructure investments, such as public transit, and market-based strategies such as congestion pricing and higher parking fees, could be a way to steer communities toward compact, mixed-use development, but, in the case of transit, would require significant new investments.
Recommendations
The report recommended that government policies to support more compact, mixed-use development should be encouraged. While the U.S. is likely to set ambitious goals to address climate change, "current development patterns will take decades to reverse," a press release about the report said.
The committee recognized that it needed more verifiable scientific evidence to fully understand how specific land-use policies might affect travel in different metropolitan areas and account for the costs and benefits of compact, mixed-use development. Given these limitations, the committee recommended five types of studies that could be conducted:
1. Federally funding longitudinal studies based on panel data to isolate the effects of different types of development patterns on travel behavior.
2. Studies of changes in metropolitan areas at finer levels of spatial detail to help inform the needs and opportunities for policy intervention.
3. Careful "before-and-after" studies of policy interventions to promote more compact, mixed-use development.
4. Studies of threshold population and employment densities to support rail and bus tranist, walking and biking. This would update old references and guide infrastructure investments, zoning and land use plans.
5. Studies of changing housing preferences and travel patterns of an aging population, new immigrant groups and young adults to determine whether future trends will differ from those of the past.
