Abstract

Findings from an empirical analysis to test the impacts of land-use mix, population density, and employment density on the use of the singleoccupant vehicle (SOV), transit, and walking for both work trips and shopping trips are presented. The hypothetical relationships tested focused on whether there is a relationship between urban form and modal choice, whether this relationship exists when controlling for non-urban form factors, whether this relationship is linear or nonlinear, and whether a stronger relationship exists between modal choice and urban form when they are measured at both trip ends as opposed to either the origin or the destination. A review of the literature and experiences suggested that a fair amount of information is known about the impacts of density on mode choice. However, considerable debate exists over whether density itself is actually the causal stimulus or a surrogate for other factors. To address this issue a data base was developed with a comprehensive set of variables for which density may be a proxy, for example, demographics and level of service. This analysis employed a correlational research design in which mode choice was compared among census tracts with differing levels of density and mix. Findings from this research indicate that density and mix are both related to mode choice, even when controlling for non-urban form factors for both work trips and shopping trips. Furthermore, the relationship between population and employment density and mode choice for SOV, transit, and walking is nonlinear for both work and shopping trips. Transit usage and walking increase as density and land-use mix increase, whereas SOV usage declines. The findings from this research suggest that measuring urban form at both trip ends provides a greater ability to predict travel choices than looking at trip ends separately. The findings also suggest that increasing the level of land-use mix at the trip origins and destinations is also related to a reduction in SOV travel and an increase in transit and walking. This research is important because of recent policy initiatives at the federal, state, and local levels that state that it is no longer feasible to maintain access to opportunities in urban areas by increasing the mobility of SOVs. Among the commonly cited reasons are economics, new environmental legislation (e.g., the Clean Air Act Amendments of 1990), public opposition, changing demographics, and political pressure to reduce fuel consumption. Several urbanform strategies have been recommended to reduce dependence on driving alone. These strategies include increasing residential and employment densities and intermixing a variety of land uses (residential, employment, and commercial). Although these strategies would seem to enhance the viabilities of alternatives to SOVs, relatively little work has been conducted to test these relationships empirically. STRUCTURE OF ANALYSIS The findings presented in this paper are based on a three-phase research approach. In Phase I information was gathered from other sources resulting in the identification of hypothetical relationships between urban form and travel behavior on the basis of theory, past research findings, and current policies. In Phase II the project data base was developed to test these hypothetical relationships. Statistical techniques were used to conduct hypothesis testing (e.g., multiple regression or correlation) in Phase III. This project is part of a research agenda developed by the University of Washington for the Washington State Department of Transportation (WSDOT). The goal of the agenda is to discover ways to plan and implement urban forms that promote increased accessibility. At the crux of this agenda are the intentions to decrease the need to travel, reduce dependence on the single-occupant vehicle (SOV), and enhance the competitiveness of other travel modes. More specifically, this paper documents empirical relationships between urban form (land-use mix and density) and trip making by individuals who use SOVs, transit, and walking as modes of travel. This analysis focuses on two trip purposes: working and shopping. POLICY PERSPECTIVE Over the past 3 years several policies that are intended to reduce the rate of growth in travel demand through the manipulation of urbanform and a variety of other factors have been enacted at the federal, state, and local levels. These policies are based on a variety of hypotheses that characterize the nature of the relationship between travel demand and both urban-form and non-urban-form factors. Although the intent of this research was to provide insight regarding urban form, it also tested and evaluated the relative impacts of nonurban-form factors. Selected policies that target both urban-form and non-urban-form factors and that affect travel demand include • The Intermodal Surface Transportation Efficiency Act of 1991, which provides new funding opportunities for non-SOV improvements;