Kicking hybrids out of carpool lane slows all traffic

UC Berkeley researchers looked at the the after effects of the state moving hybrid drivers out of diamond lanes as of July 1 — and on Bay Area freeways, the impact was significant. “Our results show that everybody is worse off with the program’s ending,” said Michael Cassidy, Berkeley professor of civil and environmental engineering. “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 the carpool lanes, the researchers explained. One factor is the presence of additional cars, including hybrids, which slow down traffic. One might think that moving vehicles out would allow the remaining cars in the lane to go faster.

But the data show that traffic speed in the carpool lane is also influenced by the speed of the adjacent lanes. Moving the hybrids into the neighboring lanes increases congestion in those lanes, which in turn slows 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 [doctoral student Kitae] 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.”

Photo source: UC Berkeley