Chapters 1 and 2 describe the dimensions of the built environment (land use) and transportation networks that are believed to affect VMT. The built environment dimensions include density, mix or diversity of land uses, concentration of development into centers, spatial arrangement of land uses, and design. The transportation network dimensions include the spatial patterns of the transportation system (whether the networks are sparse or dense, gridlike or hierarchical). 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. These dimensions are referred to in the literature as “the D’s” (see Box 3-1). A final set of characteristics—travel demand—can complement the first two, particularly through pricing.
Density is probably the most studied land use dimension, in part because it is readily measured. 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. For example, trip frequencies may increase if desired destinations are closer and easier to access. 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. VMT itself is a composite measure—the product of trip length, trip frequency, and mode choice (Ewing and Cervero 2001).
Moreover, increasing density alone may not be sufficient to lower VMT by a significant amount. 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. Development designs and street networks that provide good connectivity between locations and accommodate nonvehicular travel are important. Finally, demand management policies that complement efforts to lower VMT,