On form versus function: Will the new urbanism reduce traffic, or increase it?

A major attraction of the popular and influential planning movements known as the new urbanism, transit-oriented development, and neotraditional planning are their presumed transportation benefits. Though the architects and planners promoting these ideas are usually careful to emphasize the many ingredients necessary to obtain desired results-straightening of streets to open the local network, ''calming'' of traffic, better integration of land uses and densities, and so on--a growing literature and number of plans feature virtually any combination of these elements as axiomatic improvements. The potential problem is that the traffic impacts of the new plans are generally indeterminate, and it is unclear whether designers understand the reasons well enough to avoid unintended results. This paper proposes a simple behavioral model to identify and assess the tradeoffs these ideas impose on transportation and subdivision planners.