Kicking Hybrids from Carpool Lanes Slows Traffic

University of California at Berkeley researchers find that fewer vehicles in the carpool lane doesn’t lead to faster traffic.

by / October 12, 2011
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Transportation researchers affiliated with the University of California, Berkeley, have used roadway sensor data to come to a surprising conclusion: Discontinuing a program that gave solo drivers of hybrid vehicles access to carpool lanes has slowed traffic in all lanes.

Conventional wisdom would lead one to believe that with fewer hybrids in the carpool lane, the traffic in that lane would speed up. But that hasn’t been the case.

Everybody has slowed down — the drivers of hybrid vehicles and all other motorists on the road.

“Drivers of low-emission vehicles are worse off, drivers in the regular lanes are worse off, and drivers in the carpool lanes are worse off. Nobody wins," said Michael Cassidy, University of California, Berkeley, professor of civil and environmental engineering, in a news announcement from the university.

Cassidy and a graduate student studied six months’ worth of data from roadway sensors in the San Francisco Bay Area before and after the carpool lane privileges were revoked for hybrid cars. For one stretch of freeway in Hayward, Calif., the researchers concluded that carpool lane speeds were 15 percent slower after hybrids were expelled.


One, the researchers found that when hybrids moved back into the regular traffic lanes, those lanes were slower — and that contributed to a slowdown in the adjacent carpool lane.

"As vehicles move out of the carpool lane and into a regular lane, they have to slow down to match the speed of the congested lane," explained Kitae Jang, the doctoral student who contributed to the research. "Likewise, as cars from a slow-moving regular lane try to slip into a carpool lane, they can take time to pick up speed, which also slows down the carpool lane vehicles."

Two, in Cassidy’s words, “Drivers probably feel nervous going 70 miles per hour next to lanes where traffic is stopped or crawling along at 10 or 20 miles per hour. Carpoolers may slow down for fear that a regular-lane car might suddenly enter their lane.”

The researchers said that in order to improve traffic flow, more vehicles — not fewer — should be allowed into carpool lanes.

The researchers presented their results in a report published by UC-Berkeley’s Institute of Transportation Studies. The researchers’ paper is available here.

According to the university, in 2005 California began giving low-emission vehicles, including hybrids, a yellow sticker that qualified them to drive legally in the carpool lane. An estimated 85,000 hybrids in the state had the passes. The program was discontinued July 1 in order to comply with a federal regulation that, according to the Institute of Transportation Studies, requires low-emitting vehicles “be expelled from a carpool lane when traffic slows to below 45 mph on any portion of that lane during more than 10 percent of its operating time.”