摘要：Some claim that cardependent cities contribute to obesity by discouraging walking and bicycling. In this article, we use household activity data from the San Francisco region to study the links between urban environments and nonmotorized travel. We used factor analysis to represent the urban design and land-use diversity dimensions of built environments. Combining factor scores with control variables, like steep terrain, that gauge impediments to walking and bicycling, we estimated discrete-choice models. Builtenvironment factors exerted far weaker, although not inconsequential, influences on walking and bicycling than control variables. Stronger evidence on the importance of urban landscapes in shaping foot and bicycle travel is needed if the urban planning and public health professions are to forge an effective alliance against cardependent sprawl. URBAN PLANNERS AND PUBLIC health advocates alike decry sprawl for prodding Americans to drive their cars from anywhere to everywhere. 1, 2 Cardependent cities and suburbs, critics charge, spawn a sedentary lifestyle and associated health problems such as obesity, adding as much as $76 billion annually to US medical expenses by one estimate. 3 Eight-lane thoroughfares, serpentine roads, incomplete sidewalk networks, far-flung retail plazas, campus-style business parks, and other distinguishing traits of contemporary America are said to conspire against walking and bicycling. However, are their influences serious enough to warrant radical changes in how we design communities of the future? Numerous studies have examined the effects of built environments on motorized travel; however, far less attention has been given to impacts on walking and bicycling. 4, 5 Probing effects on nonmotorized transport requires a different analytic approach. For one thing, walking and bicycle trips are usually shorter than those made by car or public transit, requiring a finer analytic resolution. Geographic information system (GIS) tools help in this regard, especially if one knows the longitudinallatitudinal coordinates of trip origins and destinations. Additionally, choice models of motorized travel normally include comparative highway travel times of competing modes in their utility specifications. 6 This is because trip durations often vary substantially between the private car and public transit. For nonmotorized transport, and especially walking, speeds tend to be so much slower than by car, train, or bus that travel-time differentials are meaningless. Because people of a similar age and stature usually walk at comparable speeds, and given that pedestrians perceive trip making mainly in spatial terms, distance is a more suitable measure of impedance. 7 As important to the question of model specification is the inclusion of factors that represent potential barriers to walking or bicycling. 8 Besides distance, these include steep slopes, nightfall, precipitation, and less secure environs. Failure to include such factors can compromise the internal and construct validity of the research. For example, curvilinear and cul-de-sac street layouts that discourage walking are particularly common in hilly terrain. 9 Ignoring topography means that associated variables, such as road designs, that are included in a predictive model end up absorbing the influences of this omitted but relevant variable. Assigning health benefits to built environments necessitates a valid model specification that nets out impedance factors such as the presence of a steep terrain. In this study, the influences of urban designs, land-use diversity, and density patterns on the choice to walk or bicycle, vis-à-vis other factors, are examined using year-2000 data for the San Francisco Bay Area. The work builds upon other research that has applied the “3D” principle (density, diversity, and design) to associate travel choices with built environments. 10– 12 We close the article with a discussion of the public health and urban planning implications of the research findings.
