The charge of this committee was to examine the relationship between land development patterns and motor vehicle travel and to assess whether changes in development patterns—in particular, developing more compactly—can reduce energy use and carbon dioxide (CO2) emissions. A key focus of the study was the extent to which developing at higher densities would reduce vehicle miles traveled (VMT) by shortening trip lengths and making alternative modes of travel, such as transit and walking, more feasible. In response to its charge, the study committee reviewed the literature to determine what is known about the relationship between development patterns and VMT, commissioned papers to address topics that were not well covered in the literature, and developed its own scenarios to quantify the potential magnitude of VMT reductions and associated savings in energy use and CO2 emissions.
Recommendation 1: Policies that support more compact, mixed-use development and reinforce its ability to reduce VMT, energy use, and CO2emissions should be encouraged.
The committee recognizes that it does not have as much verifiable scientific evidence to support this recommendation as it would like.
The committee’s own scenarios suggest that compact development will generate only modest reductions in energy use and carbon emissions in the near term, although these savings will grow over time. By 2050, the committee’s scenarios show that reductions in VMT, energy use, and CO2 emissions resulting from compact, mixed-use development would be in the range of less than 1 percent to 11 percent, although the committee members disagreed about whether the changes in development patterns and public policies necessary to achieve the high end of these findings are plausible. Increasing densities and mixing land uses may be more achievable in some metropolitan areas than others. The examples of Portland, Oregon, and Phoenix show that concerted public policies to control and steer growth and strategic infrastructure investment can reverse current trends toward low-density new development. Without a strong state or regional role in growth management, however, the replication of these outcomes in other metropolitan areas is unlikely. Metropolitan areas differ widely in their geographic characteristics, land area, historical growth patterns, economic conditions, and local zoning and land use controls.
Nevertheless, climate change is a problem that is likely to be more easily dealt with sooner rather than later, and more energy-efficient development patterns may have to be part of the strategy if the nation sets ambitious goals to move toward greater energy efficiency and reduced production of greenhouse gases. Compact, mixed-use development also promises additional benefits in the form of increased energy efficiency of residential buildings and reduced pressure for highway construction thanks to lower growth in VMT, among other benefits. Moreover, such development need not entail the demise of single-family housing and could, if implemented carefully, reduce housing costs while increasing housing choices. The committee, however, has not examined the other benefits and costs of compact, mixed-use development or how the trade-offs among these benefits and costs might
vary by the specific types of compact development policies and the contexts in which they are applied.
Given the uncertainties, it would be wise to proceed carefully, monitoring the results and taking into account new research as it adds to the understanding of the benefits and costs that various compact, mixed-use development policies generate at different places and times. But given that the full energy and emissions benefits of land use changes take decades to realize, and current development patterns take years to reverse, it is important to start implementing these policies soon.
The committee was often stymied in its effort to identify causal linkages between land development patterns and VMT and to quantify the magnitude of effects on energy use and CO2 emissions. If land use measures are to become part of the nation’s strategy to achieve greater energy efficiency and reduce CO2 emissions, more precise estimates of the effects of such policies will be required.
Recommendation 2: More carefully designed studies of the effects of land use patterns and the form and location of more compact, mixed-use development on VMT, energy use, and CO2emissions should be conducted so that compact development can be implemented more effectively.
In particular, the committee identified five areas in which more research would be productive:
Longitudinal studies: Federally funded empirical studies based on panel data would allow better control for socioeconomic characteristics and self-selection, thus helping to isolate the effects of different
types of development patterns on travel behavior. A lack of such controls was a major shortcoming of many of the studies reviewed for this study. Most studies that find a statistically significant correlation between the built environment and VMT are cross sectional. Cross-sectional analyses that are well specified, use disaggregate data from metropolitan areas, and carefully control for socioeconomic variables and other factors that affect residential location and travel choices are valuable. Strictly speaking, however, establishing causal relationships requires a longitudinal approach that typically involves collecting panel data and following households over time to determine how a change in the built environment can lead to a change in preferences and travel behavior in the long run. Such research is time-consuming and expensive—several decades of data are probably needed to observe changes in the built environment—hence the need for sustained federal research support to collect the appropriate panel data.
Appropriate longitudinal research designs include intervention studies where a change is made to an existing community (e.g., infill at higher densities) and studies that track residents or households that move from one type of community (e.g., driving-oriented suburban) to another (e.g., more walkable and bicycle-friendly). In both cases, the main objective is to determine how these changes affect the travel behavior of residents over time. Secondarily, however, these designs help sort out the role of (possibly evolving) attitudes in understanding the impacts of changes in the built environment on travel behavior. One may find, for example, that it is mainly people with a prior disposition toward changing their behavior (but who were “stuck” in the “wrong” neighborhood before, for other reasons) who will be affected by a change in their built environment. Alternatively, one may find that only certain types of people are amenable to changes in attitude (with corresponding changes in behavior) following a change in the built environment. For either type of study, it is important
to collect baseline data (preferably on attitudes as well as behavior), ideally before the change occurs.1
Studies of spatial trends within metropolitan areas: Studies that track changes in metropolitan areas at finer levels of spatial detail over time (e.g., the evolution of employment centers and changing patterns of freight distribution) would help determine the need and opportunities for policy intervention. This study has focused primarily on residential locations in metropolitan areas and on personal travel because these are the focus of most studies and travel surveys. As noted in Chapters 2 and 3, however, travel patterns are also influenced by the locations of employment in a region (jobs–housing balance) and their concentration in employment centers outside the central business district, particularly the transit-supportive density of commercial development at the job end of the daily commute. Changes in the spatial distribution of employment within metropolitan areas and the development of new agglomerations or suburban employment centers are difficult to identify directly with current data sources.
The mechanisms by which more compact, mixed-use development could affect truck travel and logistics patterns in metropolitan areas are also poorly understood. A paper on the subject commissioned by
this committee found no relevant studies. More research is needed on topics such as the development of urban freight villages where workers live near jobs and commercial centers locate near airports. Simulations of different urban land use patterns and their effects on freight and commercial truck VMT could also be useful, including studies of specific urbanized areas.
Before-and-after studies of policy interventions to promote more compact, mixed-use development: Careful evaluations of pioneering efforts to promote more compact, mixed-used development would help determine what works and what does not. As described in Chapter 4, the landmark California legislation, passed in September 2008, to curb greenhouse gas emissions statewide through land use controls as well as technological measures (e.g., changing automotive power trains and reducing the carbon content of fuel) is an obvious example. That legislation promotes sustainable community strategies, that is, more compact land use patterns coupled with transit investments, with the objective of reducing automobile trip lengths by bringing people closer to destinations and providing alternative transportation modes. State air pollution regulators have been charged to work with metropolitan planning organizations to develop emissions reduction targets. Statewide targets have already been set for 2020. Baseline data should be collected soon to support the conduct of before-and-after evaluations of the wide range of approaches California metropolitan areas are likely to enact in response to new regulations.
Studies of threshold population and employment densities to support alternatives to automobile travel: The effectiveness of transit-oriented development, discussed in Chapter 3, depends heavily on well-designed development near well-located transit facilities. One of the seminal studies of the densities necessary to support transit (Pushkarev and Zupan 1977) is more than 30 years old. New studies of threshold densities and more data on appropriate catchment areas to support both rail and bus transit are needed to help guide transit infrastructure investments, as well as zoning and land use plans around
stations.2 Similar threshold information and data are needed to determine what development densities and land use patterns are optimal to support walking and bicycling. In contrast to transit use, encouraging more pedestrian and bicycle travel appears to depend more on neighborhood land use design and the presence of local shopping (TRB 2005).
Studies of changing housing and travel preferences: Studies of the housing preferences and travel patterns of an aging population, new immigrant groups, and young adults are needed to help determine whether future trends will differ from those of the past. Part of the difficulty of estimating reductions in VMT, energy use, and CO2 emissions from more compact development stems from the uncertainties involved in forecasting the residential location preferences and travel patterns of the population by 2050. For example, the baby boom generation will begin to sell off its large supply of low-density suburban housing within the next decade, but how will they downsize—to smaller-lot single-family units in suburban retirement communities or to apartments in more central walkable locations? And to what extent will they drive less relative to preceding cohorts of retirees? Monitoring the location preferences and travel behavior of this large group is critical in identifying opportunities for more compact development and alternatives to automobile travel. Similar monitoring of the residential preferences and travel behavior of immigrant populations and young adults, about which little is known, is also important. Finally, it would be useful to collect more data on how residents trade off travel and housing costs in making residential location decisions, particularly in the effort to find affordable housing.
If ambitious goals to reduce energy use and CO2 emissions necessitate changes in land development patterns, the research outlined above should provide a more precise understanding of future household
preferences and travel behavior that could be shaped and supported by public policy interventions (e.g., growth management policies, zoning changes) and targeted infrastructure investments.
TRB Transportation Research Board
Frank, L. D., and G. Pivo. 1994. Impacts of Mixed Use and Density on Utilization of Three Modes of Travel: Single-Occupant Vehicle, Transit, and Walking. In Transportation Research Record 1466, Transportation Research Board, National Research Council, Washington, D.C., pp. 44–52.
Ker, I., and S. Ginn. 2003. Myths and Realities in Walkable Catchments: The Case of Walking and Transit. Road and Transport Research, June.
Kittelson and Associates, Inc., KFH Group, Inc., Parsons Brinckerhoff Quade and Douglass, Inc., and K. Hunter-Zaworski. 2003. TCRP Report 100: Transit Capacity and Quality of Service Manual, 2nd ed. Transportation Research Board of the National Academies, Washington, D.C.
Pushkarev, B. S., and J. M. Zupan. 1977. Public Transportation and Land Use Policy. Indiana University Press, Bloomington.
TRB. 1995. TCRP Research Results Digest No. 7: An Evaluation of the Relationships Between Transit and Urban Form. National Research Council, Washington, D.C.
TRB. 2005. Special Report 282: Does the Built Environment Influence Physical Activity? Examining the Evidence. National Academies, Washington, D.C.