The traffic inducement effect: its meaning and measurement

The "traffic inducement effect" of road improvements in urban areas is the subject of continuing controversy. Whether, to what extent, and under what conditions adding road capacity engenders traffic growth are, on the surface, empirical questions, but they have strong ideological overtones. These derive in part from the salience of the question to fundamental, highly contentious, issues in highway policy. The ideological dimension is enhanced because definitive answers to these questions have proved illusive, a consequence of our inability to conduct the relevant controlled experiments. This paper seeks to contribute to the ongoing debate about the relationship between road supply and traffic in several ways. First, the paper reflects upon the policy context of the debate. Second, it seeks to make the questions in dispute more precise by defining metrics that capture the impact of road supply on road traffic. Third, the paper reports on research that has attempted to measure these impacts. Fourth, and finally, the paper offers recommendations for improving our ability to monitor and document on an ongoing basis how road improvements, and perhaps other transportation investments as well, influence traffic and travel in urban regions.