Building additional roadway capacity—via constructing entirely new roadways or extending or adding lanes to existing roadways—is often proposed as a solution to traffic congestion and even as a way to reduce greenhouse gas (GHG) emissions. The logic for the latter is that increasing roadway capacity increases average vehicle speeds, which improves vehicle fuel efficiency and reduces per-mile emissions of GHGs and local air pollutants. But that logic relies on the flawed assumption that the amount that people drive does not change when the time it takes to drive places changes. In fact, the amount that people drive does respond to changes in driving times. Empirical research demonstrates that as roadway supply increases, vehicle miles traveled (VMT) generally does, too. This is the “induced travel” effect—a net increase in VMT across the roadway network due to an increase in roadway capacity, which ultimately erodes any initial increases in travel speeds and causes increased GHG emissions. Researchers at the University of California, Davis reviewed the empirical research on induced travel to understand the likely effects of adding roadway capacity in a variety of contexts.
