Population Density and Travel in U.S. Cities

Project summary:

Large U.S. cities exhibit a surprising degree of variation both in population density and in the amount of daily auto travel per person. While there is a widespread belief that higher density facilitates lower driving rates (because destinations are closer together and alternate modes are more feasible), the extent and nature of this relationship are not yet well understood. In a cross-section of cities, high population density is weakly related to low daily vehicle travel; however, it is not clear to what extent lower travel is the result of density per se, as opposed to other factors, such as congestion or poverty, that may also be correlated with density. There are really two different questions that need to be answered to understand this phenomenon. First, how does population density influence travel behavior, in terms of mode choice, trip lengths and quantities, and other factors? Second, what are the sources of the large differences in daily auto travel across cities, considering that factors such as demographic and economic differences could play large roles? Comparing the answers to these two questions should then make it possible to isolate the degree of influence that density has, and also the mechanisms through which it operates.