by Kimberly Burnett, Emily Holt, Lianne Fisman, and Grace Chang
The consensus of the scientific community is that human activity has contributed substantially to limate change through increased greenhouse gas (GHG) emissions. Given the potential impacts of a continuation of these trends, this conclusion suggests that significant action is warranted to reduce GHG emissions to avoid the worst possible consequences. One proposed course of action is to increase residential density, primarily on the grounds that it will reduce vehicle miles traveled, a measure that is closely related to the GHG emissions from driving.
Much of the vast volume of research conducted on the topic of residential density and its relationship to travel shows that there is a link between residential density and the number of vehicle miles traveled. However, the relationship is complex and characterized by inter-relationships that researchers are still in the process of disentangling. On the surface, there is a clear correlation between residential density and GHG emissions. Causation is far murkier, and this review of nearly 200 studies demonstrates
that this relationship is affected by a complex set of interactions between density and at least a dozen factors, such as socioeconomic characteristics of residents, the availability of public transit, neighborhood accessibility to jobs and services, and the time and cost of various forms of transit. Although newly emerging research approaches are beginning to clarify the relationships, they are relatively untested. This review of literature on residential density and its relationship to climate change—largely via its relationship to travel behavior—is intended to help inform this aspect of the debate on climate change by summarizing and synthesizing the literature in several key areas, discussed below.
Density is thought to influence travel behavior along several different pathways. The mode used for work travel – private auto, walking, biking, or public transit – is influenced by density at both home and work. People take non-work trips, which comprise the large majority of both trips and VMT, in order to engage in activities such as personal business, shopping, socializing, and recreation. Travel is an important part of the decision to engage in these activities, and density influences these decisions in at least three ways. Density affects the quality of the travel experience (particularly for walking), the distance required to access activities, and the price of travel, both in terms of time and money.
The research on the relationship between density and travel is virtually unanimous: after controlling for socioeconomic factors, density directly influences VMT and mode choice. However, the weight of the evidence suggests that the effect of density on travel behavior is modest (roughly 5 percent reductions in VMT and vehicle trips with a doubling of density). In comparison, large increases in regional accessibility (accessibility to regional centers), are found to have a much larger impact on travel behavior – roughly 20 percent reductions in VMT.
Based on the modest impacts on VMT of increasing density—and the difficulty of achieving that added density—several researchers suggest that it is not an effective policy tool. But some research suggests that doubling density in combination with other policies, including those that affect land-use diversity, neighborhood design, access to transit, and accessibility, could have more significant impacts on travel behavior – such as reductions in VMT on the order of 25 to 30 percent. It is important to note, however, that VMT savings will be slow to develop because of the durability of the housing stock.
Self selection is an important methodological issue that affects all studies of the relationship between travel behavior and the built environment. Researchers long assumed that characteristics of the built environment, such as density, the mix of land uses, transit availability, and neighborhood design, have a causal impact on travel behavior, the source of a significant share of the nation’s GHG emissions. More recently, researchers have re-considered the direction of causality and acknowledge that land use patterns may facilitate travel behavior but not cause it, because household decisions about residential location—and all the characteristics of this location—are simultaneous with decisions about travel behavior. That is, people who dislike driving may self-select to live in walkable neighborhoods with convenient access to transit, while people who like driving may be more likely to select neighborhoods with good auto accessibility.
Much of the vast volume of research conducted on the topic of residential density and its relationship to travel shows that there is a link between residential density and the number of vehicle miles traveled. However, the relationship is complex and characterized by inter-relationships that researchers are still in the process of disentangling. On the surface, there is a clear correlation between residential density and GHG emissions. Causation is far murkier, and this review of nearly 200 studies demonstrates
that this relationship is affected by a complex set of interactions between density and at least a dozen factors, such as socioeconomic characteristics of residents, the availability of public transit, neighborhood accessibility to jobs and services, and the time and cost of various forms of transit. Although newly emerging research approaches are beginning to clarify the relationships, they are relatively untested. This review of literature on residential density and its relationship to climate change—largely via its relationship to travel behavior—is intended to help inform this aspect of the debate on climate change by summarizing and synthesizing the literature in several key areas, discussed below.
Density is thought to influence travel behavior along several different pathways. The mode used for work travel – private auto, walking, biking, or public transit – is influenced by density at both home and work. People take non-work trips, which comprise the large majority of both trips and VMT, in order to engage in activities such as personal business, shopping, socializing, and recreation. Travel is an important part of the decision to engage in these activities, and density influences these decisions in at least three ways. Density affects the quality of the travel experience (particularly for walking), the distance required to access activities, and the price of travel, both in terms of time and money.
The research on the relationship between density and travel is virtually unanimous: after controlling for socioeconomic factors, density directly influences VMT and mode choice. However, the weight of the evidence suggests that the effect of density on travel behavior is modest (roughly 5 percent reductions in VMT and vehicle trips with a doubling of density). In comparison, large increases in regional accessibility (accessibility to regional centers), are found to have a much larger impact on travel behavior – roughly 20 percent reductions in VMT.
Based on the modest impacts on VMT of increasing density—and the difficulty of achieving that added density—several researchers suggest that it is not an effective policy tool. But some research suggests that doubling density in combination with other policies, including those that affect land-use diversity, neighborhood design, access to transit, and accessibility, could have more significant impacts on travel behavior – such as reductions in VMT on the order of 25 to 30 percent. It is important to note, however, that VMT savings will be slow to develop because of the durability of the housing stock.
Self selection is an important methodological issue that affects all studies of the relationship between travel behavior and the built environment. Researchers long assumed that characteristics of the built environment, such as density, the mix of land uses, transit availability, and neighborhood design, have a causal impact on travel behavior, the source of a significant share of the nation’s GHG emissions. More recently, researchers have re-considered the direction of causality and acknowledge that land use patterns may facilitate travel behavior but not cause it, because household decisions about residential location—and all the characteristics of this location—are simultaneous with decisions about travel behavior. That is, people who dislike driving may self-select to live in walkable neighborhoods with convenient access to transit, while people who like driving may be more likely to select neighborhoods with good auto accessibility.
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