The Role of Density in Supporting Sustainable Modes: A New Perspective on the Interaction Between Urban Form and Transit Travel

In recent years, urban policies intended to reduce presumed negative externalities associated with suburbanization have focused on reducing auto travel by manipulating urban form to reduce trip frequencies and travel distances. In addition, it is assumed that shorter distances provide added opportunities to link more destinations in a single trip chain. The effectiveness of sustainable transport strategies, however, provides mixed evidence. This is so because the research is based on ad-hoc empirical specifications, lacking a formal behavioral framework that considers travel the result of activities planned and executed through space and time. To assess these shortcomings, the authors present an analytical model of the interaction between urban form and the demand for transit travel, in which residential location, transit demand, and the spatial dispersion of non-work activities are endogenously determined. Theoretically derived hypotheses are empirically tested using a dataset that integrates travel and land-use data. The authors find that population density does not have a large impact on transit demand and that the effect decreases when residential location is endogenous. When population density and residential location are jointly endogenous, the elasticity of transit demand with respect to walking distance to a transit station decreases by about 33 percent over the case in which these variables are treated an exogenous. The authors find that households living farther from work use less transit and that trip-chaining behavior explains this finding. Households living far from work engage in complex trip chains and have, on average, a more dispersed activity space, which requires reliance on more flexible modes of transportation. Therefore, reducing the spatial allocation of non-work activities and improving transit accessibility at and around subcenters would increase transit demand. Similar effects can be obtained by increasing the presence of retail locations in proximity to transit-oriented households. Although focused on transit demand, the framework can be easily generalized to study other forms of travel.