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Changing Lanes

A study of peak-hour traffic along the thousand-plus miles of high-occupancy vehicle routes in California shows that carpooling may not be the best solution for solving the state's — not to mention the nation's — traffic congestion problem.
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Next time you’re in the carpool lane, breezing past lines of traffic crawling along to your right, you might just want to hold that smirk of satisfaction: The advantages of the high-occupancy vehicle (HOV) system may be much more modest than you think.

That’s according to a recently published study by Jaimyoung Kwon, from the Department of Statistics, California State University, East Bay, and Professor Pravin Varaiya, at the University of California, Berkeley’s Department of Electrical Engineering.

They collected peak-hour traffic data at more than 700 points along California’s almost 1,200 miles of HOV routes to measure the effectiveness of the lanes that give priority to hybrid vehicles and those carrying two or more passengers.

In a study that could have implications for transportation policies in some of the nation’s major metropolitan areas, they found only limited benefits from the HOV system and questioned the rationale for expanding the network.

HOV lanes first began appearing almost 40 years ago in an effort to move more people faster along the freeway system by reducing congestion, saving time and encouraging carpooling. Decreasing emissions is another goal.

But when Kwon and Varaiya measured how well the system is meeting these targets (other than emissions), they found relatively small savings. “Travel time savings do not provide a statistically significant carpooling incentive,” they said, though they did find HOV travel times more reliable.

The research, which appeared in the February 2008 edition of the journal Transportation Research, showed carpool lanes are underused at some peak times and so congested at others that traffic is restricted to below 45 mph. This apparent paradox is largely explained by drivers switching between HOV and other lanes depending on traffic flow.

The study found that on an efficiently operating freeway, four general-purpose lanes carry more people per hour and more vehicles per hour than the configuration of one HOV lane and three unrestricted lanes.

Despite this, Kwon and Varaiya found HOV lanes can still contribute to a “well-managed freeway system” especially when they carry a significant number of buses or vanpools and where there are dual HOV lanes.

Ming Shiun Lee, a Minneapolis-based consultant working on behalf of the Department of Transportation, Federal Highway Administration, told as a “rough estimate” the U.S. has about 2,800 miles of HOV lanes.

These are concentrated in some 30 major metropolitan areas, including Houston and Dallas, Seattle, Newark and New York City, the Los Angeles and San Francisco Bay areas, and the region covering northern Virginia, Washington, D.C., and Maryland.

Kwon and Varaiya noted the California Transportation Commission has authorized plans for an additional 245 HOV lane-miles, with construction to begin by 2012 at an estimated cost of between $10 million and $30 million per mile, depending on location.

They say proposed expansion in the San Francisco Bay Area is based on the premise that “carpooling, vanpooling and express bus services have become increasingly more important to meeting the mobility needs of the region.”

That premise seems false, according to the researchers who quote a 2007 Census Bureau report signaling a drop in carpooling between 2000 and 2005, despite increasing gas prices.

Kwon and Varaiya conclude the appeal of HOV travel is weakening, and carpooling is unlikely to grow.

“A far more cost-effective solution,” they said, “is to work toward an efficient freeway system.”