May 2, 2019
Most highways in urban areas experience some degree of congestion, which reduces mobility, imposing longer travel time for drivers. In response, many cities have chosen to expand their highway infrastructure by building additional lanes or fixed-rail transit lines. This Research in Focus piece explains why highway expansion has, at best, only a limited impact on congestion. It begins by reviewing the existing literature on this topic and concludes with policy recommendations that explore better ways to address congestion, such as variable pricing.
Benefits of expansion may be limited
Because expanded capacity attracts more drivers, any congestion relief from expanding highway capacity may be temporary or limited. Expanding highway capacity increases total traffic volume, which does improve community well-being, but it is unlikely to be an efficient solution to major highway congestion issues.
How Induced Travel Demand Works
The potential for a tolling system
An alternative approach to highway expansion would be a variable tolling system. Variable tolls charge drivers a higher fee to use a highway when it is congested than when it is not. In places where variable tolls have been used, they have been shown to be an efficient tool to manage traffic flows.