Kicking Hybrids Out of Carpool Lanes Backfires

Kicking solo drivers of hybrid cars out of the carpool lanes has slowed traffic for all vehicles, according to a new study by UC Berkeley researchers.

In 2005, California granted carpool-lane priveleges to solo hybrid drivers to encourage the purchase of low-emission cars. That program ended July 1.

By 2011, 85,000 low-emission vehicles had gotten the coveted yellow stickers that gave entry to the carpool lanes, according to the report, but critics of the perk argued that solo drivers of hybrid cars were clogging up the lanes for carpoolers.

Michael Cassidy, a UC Berkeley professor of civil and environmental engineering, and Kitae Jang, a doctoral student in civil and environmental engineering, presented their analysis in a new report released by ITS, according to a UC Berkeley press release, which went on to state:

“Our results show that everybody is worse off with the program’s ending,” said Cassidy. “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.”

The counterintuitive results reflect dual—and opposing—influences on traffic speed in carpool lanes, the researchers explained. One factor is the presence of additional cars, including hybrids, which slow down traffic. One might think eliminating hybrids from carpool lanes would allow the remaining cars in the lane to go faster.

But the data showed that traffic speed in the carpool lane is also influenced by the speed of adjacent lanes. Moving hybrids into neighboring lanes increased congestion in those lanes, which in turn slowed down the carpoolers.

“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 Jang. “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.”

Human nature likely plays a role too, the researchers said. “Drivers probably feel nervous going 70 mph next to lanes where traffic is stopped or crawling along at 10 or 20 mph,” said Cassidy. “Carpoolers may slow down for fear that a regular-lane car might suddenly enter their lane.”

A new program, pending federal approval in January, would allow 40,000 super-clean plug-in-hybrids or hydrogen-powered internal combustion engine vehicles to claim green clean-air vehicle stickers and enter carpool lanes.

But the researchers predict this won't be enough. They argue that freeway traffic conditions will improve for everyone by increasing, not decreasing, the numbers allowed into carpool lanes.

“I think we need to start managing carpool facilities in a smarter way, and letting those hybrids back in the carpool lane would be a good first step,” said Cassidy. “And given the way that regular-lane speeds influence carpool lanes, added efforts to alleviate congestion in regular lanes could benefit all drivers.”