The effects of added transportation capacity on travel: a review of theoretical and empirical results

(added to the original) The addition of transportation capacity affects potentially all attributes of trips made by urban residents: time of day, destination, mode, route, and linking of trips. In the long run, added capacity may influence a household's automobile ownership decision, residence, and job location choice, as well as firms' location decisions. Neither primary growth effects nor the secondary trip effects of added capacity are thoroughly understood-determining the effect of added capacity is not at all a trivial task because it is concerned with intricately and dynamically interrelated system components: transportation supply system, land use, accessibility, and travel demand. This paper presents a review of theoretical and empirical results in the literature that shed light on the effect of added transportation capacity. Tentative findings include the following: Using existing origin-destination data appears to be a very cost-effective and expeditious approach to addressing the added capacity issue, but it can be better used with more elaborate statistical methods to test behavioral theories. There is no empirical indication that added capacity generates a significant volume of induced traffic. The standard sequential procedure is capable, in principle, of forecasting diverted, transferred, and shifted traffic, although actual practice may be less than ideal. Abbreviated application of the procedure, unwarranted attempts to transfer models and extrapolation of the models to inapplicable options are unfortunately present. A better understanding of trip timing decisions and trip chaining behavior is needed. Impacts on car ownership, residential and job location choice, and land use need to be better understood and incorporated into the forecasting procedure. More widespread use of panel surveys is encouraged.