Finally, demand management policies that complement efforts to lower VMT, Development designs and street networks that provide good connectivity between locations and accommodate nonvehicular travel are important. A diversity of land uses that results in locating desired destinations, such as jobs and shopping, near housing (preferably in centers) and improved accessibility to these destinations from either home or work are also necessary. Moreover, increasing density alone may not be sufficient to lower VMT by a significant amount. VMT itself is a composite measure-the product of trip length, trip frequency, and mode choice (Ewing and Cervero 2001). Shifts to other modes, such as transit, require that transit services be available and that density thresholds be sufficient to support adequate and reliable service. For example, trip frequencies may increase if desired destinations are closer and easier to access. However, the effect of higher densities on VMT is not entirely straightforward, making it difficult to determine the net reduction in automobile use from increased densities. A final set of characteristics-travel demand-can complement the first two, particularly through pricing.ĭensity is probably the most studied land use dimension, in part because it is readily measured. These dimensions are referred to in the literature as “the D’s” (see Box 3-1). Together, the land use and transportation network measures interact to affect destination accessibility (ease of travel between trip origins and desired destinations) and distance between development and transit. The transportation network dimensions include the spatial patterns of the transportation system (whether the networks are sparse or dense, gridlike or hierarchical). The built environment dimensions include density, mix or diversity of land uses, concentration of development into centers, spatial arrangement of land uses, and design. Additional detail on the two case studies is provided in Annex 3-1.Ĭhapters 1 and 2 describe the dimensions of the built environment (land use) and transportation networks that are believed to affect VMT. The final section presents a series of findings. The potential synergies of these policies for VMT reduction are discussed next through two case studies that demonstrate what can be accomplished but also underscore the associated challenges and costs. Few of these studies, however, consider the potential effects on VMT of a package of policies that combine increased density with higher employment concentrations, improved access to a mix of diverse destinations, a good transit network, and parking charges. Drawing on a paper commissioned by the committee (Brownstone 2008) and earlier reviews of the literature, the main section of the chapter summarizes the results of the most methodologically sound studies that examine the relationship between household travel and the built environment while controlling for socioeconomic variables and other factors (e.g., attitudes, preferences) that influence travel behavior. It then examines issues related to research design and data that help explain the variability in study results. It starts with a brief discussion of the built environment–VMT connection. This chapter summarizes what is known from the literature about the effect of changes in the built environment-in particular, more compact, mixed-use development-on VMT. The congressional request for this study asks for consideration of “the correlation, if any, between land development patterns and increases in vehicle miles traveled (VMT),” implying that sprawl induces more travel. Impacts of Land Use Patterns on Vehicle Miles Traveled Evidence from the Literature
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