The vast majority of the U.S. population—some 80 percent—now lives in metropolitan areas, but population and employment continue to decentralize within regions, and density levels continue to decline at the urban fringe. Suburbanization is a long-standing trend that reflects the preference of many Americans for living in detached single-family homes, made possible largely through the mobility provided by the automobile and an extensive highway network. Yet these dispersed, automobile-dependent development patterns have come at a cost, consuming vast quantities of undeveloped land; increasing the nation’s dependence on petroleum, particularly foreign imports; and increasing greenhouse gas (GHG) emissions that contribute to global warming. The primary purpose of this study is to examine the relationship between land development patterns, often referred to as the built environment, and motor vehicle travel in the United States and to assess whether petroleum use, and by extension GHG emissions, could be reduced through changes in the design of development patterns (see Appendix A for the full statement of task). A key question of interest is the extent to which developing more compactly would reduce vehicle miles traveled (VMT) and make alternative modes of travel (e.g., transit, walking) more feasible. The study is focused on metropolitan areas and on personal travel, the primary vectors through which policy changes designed to encourage more compact development should have the greatest effect.
The adverse effects of suburbanization and automobile dependence have long been evident but are currently of particular concern for several reasons. First, after decades of low energy prices, the cost of oil rose to record highs in 2008, reflecting the growth of China and India and the instability of many key suppliers in the Middle East and other oil-producing areas and underscoring U.S. dependence on imported fuels. The transportation sector as a whole accounts for more than 28 percent of annual U.S. energy consumption. Cars and light trucks, most of which are used for personal transportation, represent about 17 percent of that total, and this share has been rising. Second, concern about climate change continues to rise both domestically and internationally, and transportation is a major and increasing contributor to that growing problem. Gasoline consumption, largely by personal vehicles, accounts for about 20 percent of annual carbon dioxide (CO2) emissions, the largest single source of U.S. GHG emissions and the focus of the analyses conducted for this study. An additional factor, although less newsworthy, is the health risks resulting from transportation emissions and the difficulty being experienced by many regions in meeting federal clean air standards. At the same time, changing demographics—an aging population, continued immigration—and the possibility of sustained higher energy prices should lead to more opportunities for the kinds of development patterns that could reduce vehicular travel, thereby saving energy and reducing CO2 emissions.
To examine the potential for reducing VMT, energy use, and CO2 emissions through more compact development, the committee formed to conduct this study commissioned five papers to augment its members’ expertise, received informational briefings at its early meetings, and performed a review of the literature. The committee’s findings and resulting recommendations are presented below. The committee reached consensus on all but one issue—the extent to which development is likely to become more compact by 2050 (see the text following Finding 4 for a detailed discussion).
Link Between Development Patterns and VMT
Finding 1: Developing more compactly, that is, at higher residential and employment densities, is likely to reduce VMT.
Both logic and empirical evidence suggest that developing at higher population and employment densities results in closer trip origins and destinations, on average, and thus in shorter trip lengths, on average. Theory suggests that reduced trip lengths can increase trip frequencies, but empirical evidence suggests that the increase is not enough to offset the reduction in VMT that comes from reduced trip length alone. Shorter trips also may reduce VMT by making walking and bicycling more competitive alternatives to the automobile, while higher densities make it easier to support public transit. Mixing land uses to bring housing closer to jobs and shopping can reduce trip lengths as well. The committee refers to these development patterns as compact, mixed-use development.
Compact, mixed-use development can reduce VMT by differing means and amounts depending on where the development in a region occurs. Empirical data are lacking that demonstrate how specific design features applied in different contexts affect VMT. Nevertheless, at the low-density urban fringe, for example, simply reducing single-family lot sizes— say, from 1 acre to a quarter acre—should reduce vehicle trip distances by bringing origins and destinations closer together. In established moderate-density suburbs and along transportation corridors, smaller lots and multiunit housing can support public transit and encourage walking and bicycling, further reducing VMT. And in established urban cores, redevelopment of strategically located but underused parcels can support investment in rail transit.
The effects of compact, mixed-use development on VMT are likely to be enhanced when this strategy is combined with other policy measures
that make alternatives to driving relatively more convenient and affordable. Examples of such measures include a street network that provides good connectivity between locations and accommodates non-vehicular travel, well-located transit stops, and good neighborhood design. Likewise, demand management measures, such as reducing the supply and increasing the cost of parking, can complement efforts to reduce VMT.
Evidence from the Literature
Finding 2: The literature suggests that doubling residential density across a metropolitan area might lower household VMT by about 5 to 12 percent, and perhaps by as much as 25 percent, if coupled with higher employment concentrations, significant public transit improvements, mixed uses, and other supportive demand management measures.
Studies aimed at isolating the effect of residential density while controlling for sociodemographic and other land use variables consistently find that doubling density is associated with about 5 percent less VMT on average; one rigorous California study finds that VMT is lower by 12 percent. The same body of literature, mainly U.S.-based studies, reports that VMT is lower by an average of 3 to 20 percent when other land use factors that often accompany density, such as mixed uses, good design, and improved accessibility, are accounted for, and suggests further that in some cases these reductions are additive. These studies include changes in density for a range of geographic areas, from census block groups, to census tracts, to neighborhoods.
A higher VMT reduction that the committee uses as an upper bound in its own scenario analyses comes from a single but carefully done statistical analysis of metropolitan development patterns, transit service, and travel behavior. The authors of this analysis interpret its findings by using the following thought experiment. If households in Atlanta, one of the least dense metropolitan areas, were located in an area with the
residential population density, concentrated employment, extensive public transit system, and other land use characteristics of the Boston metropolitan area, VMT per household could be lowered by as much as 25 percent. Of course, the urban structure of Atlanta could not literally be converted to that of Boston because of vast differences in topography and historical development patterns. Combining density increases with transit investment, mixed uses, higher parking fees, and other measures, however, could provide the synergies necessary to yield significant reductions in VMT, even in low-density metropolitan areas like Atlanta.
Most of the above studies are subject to a number of shortcomings. For example, many fail to distinguish among different types of density changes (e.g., decreasing lot size versus increasing multifamily housing) or the location of these changes in a region. Relatively few (but including the California study mentioned) attempt to account for self-selection—the tendency of people to locate in areas consistent with their housing and travel preferences. Without doing so, one could not assume, for example, that the typical Atlanta resident who moved to an area with the characteristics of Boston would travel like the typical Boston resident, although both attitudes and behavior are likely to be influenced by the built environment over time. Finally, most studies are cross-sectional, that is, they find an association between higher density and lower VMT at a single point in time but cannot be used to infer cause and effect.
Effects on Energy and CO2Emissions
Finding 3: More compact, mixed-use development can produce reductions in energy consumption and CO2emissions both directly and indirectly.
To the extent that more compact development reduces VMT, it will directly reduce fuel use and CO2 emissions. The VMT savings will be slow to develop, however, if only because the existing building stock is
highly durable; therefore, opportunities to build more compactly are limited largely to new housing as it is built to accommodate a growing population and to replace the small percentage of existing units that are scrapped each year. Over time, moreover, if the fuel efficiency of the passenger vehicle fleet improves through either regulation (such as the new Corporate Average Fuel Economy standards) or sustained higher fuel prices that encourage consumers to purchase more energy-efficient vehicles, the savings in fuel use and CO2 emissions from developing more compactly will be reduced, all else being equal.
Additional, indirect savings in energy consumption and CO2 emissions from more compact, mixed-use development can accrue from higher ownership of smaller, more fuel-efficient vehicles; longer vehicle life times due to driving less; smaller homes and more multifamily units, which are more energy efficient than the average single-family home; and more efficient urban truck travel and delivery patterns. Savings from reduced heating and cooling needs per dwelling unit due to a higher share of multifamily units and, to a lesser extent, smaller single-family units could add significantly to the savings from VMT reductions. Over time, however, if the energy efficiency of residential heating and cooling improves, the savings in energy and CO2 emissions from shifting to multifamily or smaller single-family units will decline proportionately.
Quantification of the Effects
Finding 4: Illustrative scenarios developed by the committee suggest that significant increases in more compact, mixed-use development will result in modest short-term reductions in energy consumption and CO2emissions, but these reductions will grow over time.
The committee’s scenarios assume that compact development is focused on new and replacement housing because of the difficulty of converting any significant fraction of existing housing to higher densities. As many as 57 million new housing units are projected to accommodate
population growth and replacement housing needs by 2030, growing to between 62 million and 105 million units by 2050—a substantial net addition to the housing stock of 105.2 million in 2000. Developing more compactly is defined as doubling the current density of new residential development, mainly at the urban fringe where most new development is taking place, but also through some strategic infill. The scenario results depend importantly on assumptions about what percentage of new housing developments will be built compactly and how much less residents of these new, more compact developments will drive. The scenarios do not account for any behavioral feedbacks, but the sensitivity of key assumptions is tested.
In an upper-bound scenario that represents a significant departure from current conditions, the committee estimates that steering 75 percent of new and replacement housing units into more compact development and assuming that residents of compact communities will drive 25 percent less would reduce VMT and associated fuel use and CO2 emissions of new and existing households by about 7 to 8 percent relative to base case conditions by 2030, with the gap widening to between 8 and 11 percent less by 2050. A more moderate scenario, which assumes that 25 percent of new and replacement housing units will be built in more compact developments and that residents of those developments will drive 12 percent less, would result in reductions in fuel use and CO2 emissions of about 1 percent relative to base case conditions in 2030, growing to between 1.3 and 1.7 percent less than the base case in 2050. If the residents of compact developments drive only 5 percent less—the lower bound of available estimates—the savings in fuel use and CO2 emissions would be less than 1 percent compared with the base case, even in 2050. Thus, the committee believes 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 by 2050, although the committee disagreed about whether the changes in development patterns and public policies necessary to achieve the high end of these findings are plausible.
All scenarios increase the density of development and thus represent a departure from current trends. New development in metropolitan areas has occurred at lower than average densities for decades. Nevertheless, doubling the density of 25 percent of new development is possible, particularly by 2050. Average densities for new development would not be higher than the average density of development that existed in 2000, and precedents for higher densities through smaller lot sizes and infill development near major transportation corridors can be found in growing areas such as Phoenix and Portland. Doubling the density of 75 percent of new development by 2050 would be much more challenging. It would require, for example, curtailing most large-lot development or adding a significant proportion of new development as infill to achieve densities above current levels and substantially above a 2050 baseline of continuing low-density development.
The committee disagreed about the feasibility of doubling the density of 75 percent of new development, even by 2050. Those members who believe it possible question whether densities will keep declining. Macroeconomic trends—likely higher energy prices and carbon taxes—in combination with growing public support for strategic infill, investments in transit, and higher densities along rail corridors could result in considerably higher densities by 2050. Other members believe that the curbing of large-lot development at the urban fringe or substantial infill entailed in the upper-bound scenario requires such a significant departure from current housing trends, land use policies of jurisdictions on the urban fringe, and public preferences that those measures are unrealistic absent a strong state or regional role in growth management.
Obstacles and Opportunities
Finding 5: Promoting more compact, mixed-use development on a large scale will require overcoming numerous obstacles. These obstacles include the traditional reluctance of many local governments to zone for such
development and the lack of either regional governments with effective powers to regulate land use in most metropolitan areas or a strong state role in land use planning.
Local zoning regulations—particularly suburban zoning that restricts density levels and the mixing of land uses—represent one of the most significant barriers to more compact development. Highly regulated land use markets also limit the supply of compact developments, despite evidence of increased interest in such communities. Land use control is, and has remained, largely a local government function and thus sensitive to local concerns. These local concerns—about congestion, for example, or local taxes or home values—are understandable and legitimate even though they sometimes conflict with other understandable and legitimate regional or national concerns, such as housing affordability or global warming. Land use policies aimed at achieving sweeping changes in current development patterns are thus likely to be impeded by political resistance from existing homeowners and local governments that reflect their interests. This political resistance may help explain why metropolitanwide or state policies aimed at controlling land use and steering development and infrastructure investments are not widespread. It is also the reason why the committee characterized as an upper bound the scenario in which 75 percent of new development is compact.
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. Coordinated public infrastructure investments and development incentives can be used to encourage more compact development in these locations, and zoning regulations can be relaxed to steer this development to areas that can support transit and nonmotorized travel modes. Market-based strategies, such as congestion pricing and market-based parking fees,
along with zoning requirements for maximum rather than minimum parking, can complement higher-density development patterns that encourage transit use and pedestrian travel. The Portland, Oregon, and Arlington, Virginia, case studies described in this report demonstrate how the application of these policies has led the real estate market to respond with more compact, mixed-use development. In the longer term, if housing preferences and travel patterns change and compact, mixed-use developments become more commonplace, a greater political consensus may emerge in support of stronger state and regional measures to control land use. Policy instruments might include setting urban growth or greenbelt boundaries to steer growth to areas already developed.
Other Benefits and Costs
Finding 6: Changes in development patterns significant enough to substantially alter travel behavior and residential building efficiency entail other benefits and costs that have not been quantified in this study.
On the benefit side, more compact, mixed-use development should reduce some infrastructure costs, increase the feasibility and cost-effectiveness of public transit, and expand housing choices where compact developments are undersupplied. Other benefits include less conversion of agricultural and other environmentally fragile areas and greater opportunities for physical activity by facilitating the use of nonmotorized modes of travel, such as walking and bicycling.
On the cost side, the savings in highway infrastructure will be offset, at least in part, by increased expenditures for public transit, particularly rail transit, to support high-density development. As noted earlier, moreover, many Americans appear to prefer detached single-family homes in low-density suburbs that are often associated with more privacy, greater access to open space and recreation, and less noise than characterize many urban neighborhoods. Of course, housing
preferences may change in the future with changes in the demographic and socioeconomic characteristics of the population. Moreover, as suggested above, well-designed compact, mixed-use developments may currently be undersupplied because of exclusionary suburban zoning.
RECOMMENDATIONS FOR TAKING ACTION
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. Moreover, the committee 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. 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 GHGs. Compact development also promises benefits in the form of reduced pressure for highway construction due to lower growth in VMT. Moreover, compact development does not entail the demise of single-family housing and may, if implemented carefully, reduce housing costs while increasing housing choices.
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 will take decades to realize and current development patterns will take years to reverse, it is important to start implementing these policies soon.
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. Use of longitudinal panel data is the only way to determine how a change in the built environment can lead to a change in preferences and travel behavior in the long run.
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 subcenters and changing patterns of freight distribution) would help determine the needs and opportunities for policy intervention.
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. The landmark California legislation to reduce urban sprawl and automobile travel offers an obvious example; baseline data should be collected soon so before-and-after evaluations can be conducted.
Studies of threshold population and employment densities to support alternatives to automobile travel: Studies of the threshold densities required to support rail and bus transit would help guide infrastructure investments as well as zoning and land use plans around stations. Current rules of thumb are based on outdated references. Similar threshold information is needed to determine what development densities and land use patterns are optimal to support walking and bicycling.
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.