New Carpool Rule Slows Traffic for All

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FALL 2011— If you were a solo driver of a hybrid who once enjoyed access to California’s carpool lanes and now you’re stuck in a slow-moving regular-use lane, you’re probably unhappy. Your commute has gotten a lot slower.

But here’s some consolation: the carpool lanes are moving more slowly, too. Even with fewer cars in them.
 
New research by Michael Cassidy, Professor of Civil and Environmental Engineering, and ITS Ph.D. student Kitae Jang shows that kicking single occupant hybrids out of carpool lanes has backfired, slowing traffic not only in regular-use lanes, but also in the diamond lanes reserved for carpoolers and solo vehicles equipped with white clean air vehicle stickers as of July 1, 2011. The white stickers are available only for hydrogen fuel cell, 100 percent battery electric, and compressed natural gas vehicles.
 
To comply with a provision of the federal Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users—better known as SAFETEA-LU—the state booted some 85,000 solo drivers out of the carpool lanes. 
 
The regulation requires that low-emitting vehicles like the popular hybrids 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. States are currently scrambling to evaluate their freeway carpool facilities to determine whether they meet the new federal standards.
 

A counterproductive regulation

 
“In this situation we have two effects working in opposite directions,” explained Cassidy. “One is very intuitive: it says that as you get cars out of the carpool lane, those that remain travel faster. Okay, that’s good, and that was the reason, I guess, for the regulation.”
 
But unless those vehicles simply “disappear into thin air,” he said, they’ll move into adjacent regular-use lanes where their presence will increase congestion in those lanes. And when that happens, traffic will also slow down in the carpool lane—even if fewer vehicles are using that lane.
 
There are at least two possible reasons for this phenomenon, said Jang, who has studied HOV lanes for several years and is preparing his dissertation on the subject.
 
“We know from previous work done here at ITS that one is physical—as vehicles move out of the carpool lane from a regular lane, they must slow considerably to move into a congested neighboring lane,” he explained. “And as cars from a slowly-moving regular lane try to slip into a carpool lane, they slow down carpool lane vehicles.”
 
The other is likely psychological. “Drivers don’t like going 70 mph next to lanes where traffic is stopped or moving very slowly,” he said. “It’s scary. So they slow down.”
 
The researchers have observed this phenomenon not only on Bay Area freeways, but also in southern California, where carpool and regular-use lanes are separated by a solid line, and drivers may enter and exit the lanes only where marked. They have shown, too, that the same sort of carpool lane slowing occurs in South Korean bus-only lanes.
 

Test case in Hayward

 
In their paper, Dual Influences on Vehicle Speed in Special-Use Lanes and Critique of US Regulation, the two researchers first predicted the counterproductive outcomes of the new requirement by applying kinematic wave analysis, a technique fundamental to traffic flow theory.
 
They chose a four-mile stretch of I-880 in Hayward between the Alvarado Niles Road and Tennyson Road on-ramps where a bottleneck arises at the downstream end of the site during each rush period. Speeds in the carpool lane fall below 45 mph for more than 35 percent of the time the carpool lane is operating.
 
If any carpool facility in the Bay Area would benefit from the new regulation, it should be this one: for reasons unknown, the carpool lane along this stretch is less sensitive to congestion in adjacent lanes and responds better to reduced flow—or fewer cars—than other carpool facilities.
 
But the researchers' models predicted that the regulation could increase a rush period’s “people- hours-traveled” by more than 400 person-hours, a 12 percent increase. Similarly, total rush-period vehicle-hours-traveled could increase by about 11 percent.
 
“The Hayward site’s commute conditions would improve, on the whole, not by tightening the carpool lane’s restrictions, but by easing them somewhat so that more vehicle classes would enjoy access to that lane,” the researchers wrote.
 

Real data confirmation

 
Cassidy and Jang tested their predictions against real data measured by loop detectors on the Hayward Stretch before and after the regulation took effect. They found carpool lane speeds had actually dropped much as they had predicted.
 
Given that the Hayward carpool lane on this particular stretch of I-880 showed greater chance of improving under the new regulation than elsewhere, the researchers are concerned that even more damage in terms of slower speeds will occur in other carpool lane facilities.
 
In their paper, the researchers argue that the regulation’s objective—to maintain carpool speeds at or above 45 mph for 90 percent of its operating hours—is “off-target.”
 
“Slow carpool speeds do not necessarily indicate that the lane is over-used,” explained Cassidy.
 
“The literature indicates that carpool lanes’ attractiveness to commuters is based less on the magnitude of its speed than on the quality of travel that it provides relative to that of the adjacent regular-use lanes,” the researchers wrote.
 
“Everyone suffers under the new regulation. I think we need to start managing carpool facilities in a smarter way.”
 
In addition to allowing more classes of vehicles to utilize carpool lanes, the authors propose improving traffic flow in regular lanes through various means, including discouraging lane-changing maneuvers in areas of bottlenecks. They also suggest siphoning more regular-use drivers into high occupancy lanes, or devising a policy that would enable commuters to take turns using special lanes.
 
Currently, only federally-approved "Inherently Low Emission Vehicles," or ILEVs, such as hydrogen fuel cell, 100 percent battery electric, and compressed natural gas vehicles, are eligible to receive the new white clean air vehicle stickers, which allow solo drivers into the carpool lanes. Some 14,000 vehicles in California have qualified, and it is not expected the numbers will increase dramatically.
 
A new program, pending federal approval next January, will allow 40,000 super-clean plug-in-hybrids or hydrogen-powered internal combustion engine vehicles to claim new green clean air vehicle stickers and enter carpool lanes beginning in 2015.
 
Over time, maybe some of those 85,000 hybrid drivers will switch to cleaner vehicles and move back into the carpool lanes. But for now, their presence in the in the regular-use lanes is creating more, not less, congestion.
 
“Everyone suffers under the new regulation,” said Cassidy. “I think we need to start managing carpool facilities in a smarter way.”
 
Read UC Berkeley press release here.

 -- Christine Cosgrove

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