March 14, 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 examines the costs of expanding highway capacity as well and how the unintended consequences of doing so may actually increase demand.
Benefits of expansion may be limited
Because expanded capacity attracts more drivers, any congestion relief from expanding highway capacity may be temporary or limited:
- When congested highways are expanded, driving speeds increase, at least temporarily, saving travel time for drivers using the highway.
- Lower travel time results in changes in the behavior of commuters around the previously congested highway. Drivers take more trips. Some commuters who have been using alternate, less-desirable routes or public transit may switch to using expanded highways.
- This is called induced travel. As a result, all or most of the congestion returns over time.
While expanding highway capacity increases total traffic volume, which does improve community well-being, it is unlikely to be an efficient solution to major highway congestion issues.
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 for using a highway when it is congested.
- Tolls are lower during periods where highways are uncongested.
- This creates an incentive for drivers to shift the timing of their trips, carpool, or use public transit, reducing congestion during rush hour.
In places where variable tolls have been used, they have been shown to be an efficient tool to manage traffic flows.