Kicking hybrids out of carpool lanes in California has slowed traffic in the state, a new study by transportation engineers at the University of California, Berkeley.
Transportation researchers from the University have found that pushing drivers out of the carpool lane has inadvertently slowed traffic across all lanes.
Back in 2005, sole occupants driving hybrid cars were granted permission to use carpool lanes along with multiple occupant vehicles, as part of measures to encourage the uptake of low emission vehicles.
By 2011, some 85,000 low-emission vehicles had applied for yellow stickers that gave them entry into the carpool lanes.
But that preferential treatment ended in July this year, following complaints from other motorists that hybrids were clogging up the carpool lanes for genuine carpoolers.
Researchers at UC Berkeley’s Institute of Transportation Studies (ITS) used traffic flow theories and six months of data from roadway sensors measuring speed and congestion along all freeway carpool lanes in the San Francisco Bay Area. They used the information to predict the impact on vehicle speed of the hybrids’ removal from carpool lanes. Additional data collected after the program’s July 1 expiration supported their predictions.
“Our results show that everybody is worse off with the program’s ending,” said Michael Cassidy, UC Berkeley professor of civil and environmental engineering who presented the research. “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.”
A carpool lane along a four-mile stretch of I-880 in Hayward in the East Bay area of San Francisco, for instance, saw a 15 per cent reduction in speed after single-occupant hybrids were expelled after July 1.
Although you might expect that the reduction in traffic in the carpool lanes would allow the remaining cars in the lane to go faster, the research shows that the removal of the hybrids actually had a counter-intuitive effect.
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 neighbouring 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,” explains Kitae Jang, a doctoral student in civil and environmental engineering. “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 miles per hour next to lanes where traffic is stopped or crawling along at 10 or 20 miles per hour,” said Cassidy. “Carpoolers may slow down for fear that a regular-lane car might suddenly enter their lane.”
