Researchers at the University of California, Berkeley and California State University, East Bay have measured the effect of high occupancy vehicle (HOV) restrictions on 100 miles of freeway in the San Francisco Bay area and found the lanes have had the opposite of their intended effect. Using detectors buried in the pavement, they analyzed four-and-a-half years worth of speed and travel time data from 2001 to 2005. Because the HOV/carpool restrictions only apply for 8-10 hours a day on the freeway segments examined, traffic flow was measured both with and without the restriction.

"HOV actuation imposes a twenty percent capacity penalty," wrote Jaimyoung Kwon and Pravin Varaiya, the study's authors. "The HOV restriction significantly increases demand on the other lanes causing a net increase in overall congestion delay. HOV actuation does not significantly increase person throughput."

The study found that at 60 MPH, an HOV lane has a maximum flow of 1600 vehicles per hour compared with 2000 for the general purpose lanes. Researchers used an estimate of 1.3 or 1.4 occupants per vehicle in the general purpose lanes to calculate that HOV lanes are not able to transport more people per hour than a general purpose lane.

Moreover, no increase in carpooling was measured as a result of increased delays in the general purpose lanes either in the short term or in a long-term analysis.

"HOV lanes exacerbate the problem," the authors conclude. They found overall congestion would be reduced by eliminating the HOV lane, but only in areas where effective on-ramp metering is employed during congested periods.

A report released last year also shows that the most common form of HOV lane, where general and restricted traffic is not separated by a physical barrier, causes a fifty percent increase in accidents. The full report is available in a 163k PDF file at the source link below.