The Demand for Travel

As described in chapter 1, the new urban designs are part philosophy, part art, part economics, and part social optimism. Still, a key to their popularity is the open embrace of conventional and even conservative standards of neighborhood form, scale, and style. Many new urban designs self-consciously recall small town settings where neighbors walk to get a haircut and stop on the way to chat with neighbors sitting on the front porch watching the kids play. The attraction of these ideas is subjective, personal, yet pervasive. After all, in principle, what is not to like about pretty homes in quiet, friendly, and functional neighborhoods? But will they improve the traffic? Chapter 3 concluded that existing evidence is unsatisfactory in several respects. Among the problems identified in the literature was the common absence of a conceptual framework for hypothesizing how urban form might be expected to influence travel behavior. In particular, only a small share of the studies in this area even attempt to model travel behavior in the conventional manner, that is, as travel demand. In this chapter, we develop a framework for consistently evaluating the net travel impacts of changing land-use patterns, such as many new urban designs propose. The idea is to adapt a simple model of travel demand to measurable urban form elements. This permits us to derive specific conclusions that follow directly from the assumptions of the model as well as specific hypotheses that can be tested only with data on observed behavior. These assumptions are summarized in figure 4.2. The last part of the chapter develops an empirical implementation of the model and these hypotheses, which is applied to data in chapter 5. The theory of demand provides perhaps the most straightforward way to analyze travel behavior, by emphasizing how overall resource constraints force trade-offs among available alternatives, such as travel modes and trip distances, and how the relative attractiveness of those alternatives in turn depends on relative costs, such as trip times. This framework assumes that individuals make choices, either alone or as part of a family or other group, based on their preferences over the goods in question, the relative costs of those goods, and available resources (e.g., Kreps, 1990).