Expert Commentary

Smoothing effect of carpool lanes on freeway bottlenecks

2008 U.C. Berkeley study looking at the effect of carpool lanes on the number of person and vehicle hours traveled.

High-occupancy vehicle lanes are intended to both speed traffic and improve air quality. This is done by dedicating a lane to cars with two or more occupants, thus reducing the number of vehicles traveling on the road. This both speeds traffic flows and reduces pollution. Some have argued, however, that if underutilized, such lanes can create congestion.

A 2008 paper by researchers from U.C. Berkeley, “The Smoothing Effect of Carpool Lanes on Freeway Bottlenecks,” finds that overall, carpool lanes have a positive effect on traffic flows. The study is based on an examination of freeway bottlenecks in California’s San Francisco Bay Area.

Key findings are:

  • At bottlenecks, carpool lanes have a smoothing effect, reducing drivers’ disruptive lane changes and helping increase the flow in non-carpool lanes.
  • Because of this smoothing effect, highways with carpool lanes reduce person hours traveled by 30% compared to highways without carpool lanes.
  • Highways with carpool lanes also reduce vehicle hours traveled by 15% compared to highways without carpool lanes.
  • Even when carpool lanes at bottlenecks are underutilized, they can reduce highways’ vehicle-hours traveled and person-hours traveled.

The authors concluded that the carpool lanes’ smoothing effect, a direct consequence of reducing drivers’ disruptive lane changes, can be induced by other means as well. These can include solidly striping highways to limit lane changes or sorting vehicles by class. These methods could be used on any highway, the authors state, whether or not it already has a carpool lane.

Tags: California, congestion, pollution, traffic, infrastructure