National Center for Sustainable Transportation
Handy, Susan L. (2015) NCST Policy Brief: Increasing Highway Capacity Unlikely to Relieve Traffic Congestion. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-15-47
Reducing traffic congestion is often proposed as a solution for improving fuel efficiency and reducing greenhouse gas (GHG) emissions. Traffic congestion has traditionally been addressed by adding additional roadway capacity via constructing entirely new roadways, adding additional lanes to existing roadways, or upgrading existing highways to controlled-access freeways. Numerous studies have examined the effectiveness of this approach and consistently show that adding capacity to roadways fails to alleviate congestion for long because it actually increases vehicle miles traveled (VMT).
An increase in VMT attributable to increases in roadway capacity where congestion is present is called “induced travel.” The basic economic principles of supply and demand explain this phenomenon: adding capacity decreases travel time, in effect lowering the “price” of driving; and when prices go down, the quantity of driving goes up. Induced travel counteracts the effectiveness of capacity expansion as a strategy for alleviating traffic congestion and offsets in part or in whole reductions in GHG emissions that would result from reduced congestion.