UC Davis Institute of Transportation Studies

The “induced travel” effect is a net increase in vehicle miles traveled (VMT) across the roadway network due to an increase in roadway capacity. Adding capacity can increase the average travel speed on the roadway (at least initially), increase travel time reliability, and make driving on the roadway appear safer or feel less stressful. It might also provide access to previously inaccessible areas. All of these effects reduce the perceived “cost” of driving. And when the cost of driving goes down, the quantity of driving goes up. Accounting for induced travel in transportation planning is important from the standpoint of accurately assessing both the benefits and costs of projects that expand roadway capacity. This brief summarizes the robust empirical evidence on the magnitude of the induced travel effect and discusses the limitations of travel demand forecasting models in fully capturing the effect.