A Review of the Representation of Induced Highway Travel in Current Travel and Land Use Models

A considerable body of research on induced travel has emerged over the last several decades, and induced travel has been acknowledged by the U.S. Transportation Research Board and Environmental Protection Agency. This has brought renewed attention to the representation of induced travel in regional land use and travel demand models. A number of case studies (Sacramento, CA, Chittenden, VT, and Salt Lake City, UT) have assessed the ability of existing travel and land use models to represent the induced travel effects of new highway capacity (or elasticity of VMT with respect to lane miles and travel time). In addition, these studies have conducted sensitivity tests, by turning on and off model components, to isolate the relative contribution with respect to the models' representation of induced travel. The results indicate that when travel times are fed back to a land use model and/or the trip distribution step, then (1) models can represent induced travel within the range documented in the empirical literature and (2) the effect of new highway capacity on land use and trip distribution can significantly contribute to the model's representation of induced travel. If induced travel is not represented in travel and land use models, then the need for, and the benefit of, a highway project will tend to be overstated (e.g., 16% to 236% of vehicle hours traveled), and negative environmental effects will be understated (e.g., 72% to 192% of NOx vehicle emissions).