On July 1, 2011 the state of California decided to revoked carpool privileges for hybrid and electric vehicles. They were hoping to decrease traffic congestion in these High Occupancy Vehicle (HOV) Lanes as the amount of hybrid and electric vehicles on the roads increased. According to a study done by researchers at UC Berkeley Institute of Transportation, the additional vehicles in the regular traffic lanes has slowed all traffic substantially.

Using traffic flow theories and six months of data from roadway sensors measuring speed and congestion along all freeway carpool lanes in the San Francisco Bay Area, researchers found that the slower traffic in the regular lanes made it more difficult for the carpool drivers to migrate in and out of the HOV lanes. The result was an increase of traffic congestion in all lanes. The study suggests that the state should reconsider their decision, and allow the hybrid and electric vehicles back in the carpool lanes.