On 101 in Palo Alto and Mountain View, a 2.6-mile stretch of double carpool lanes in each direction is expected to open in 2013. The expansion will start at state Highway 85 and stretch to Oregon Expressway.
But at least one University of California Berkeley study calls into question the benefit of carpool lanes, finding that carpool lanes are only negligibly efficient. The average travel-time savings was 1.7 minutes over 10 miles, with the median at 0.7 minutes, researchers Jaimyoung Kwon and Pravin Varaiya wrote in their 2007 study, "Effectiveness of California's High Occupancy Vehicle (HOV) system."
Kwon was formerly at the Department of Statistics at California State University in Hayward and is now tech director at AOL, and Varaiya is a professor at the University of California Berkeley Department of Electrical Engineering and Computer Sciences.
After taking random 10-mile samples based on Caltrans traffic data, the researchers noted most carpool lanes failed Caltrans' goal of 5 to 10 minutes in saved time. Only 15 percent of these routes offered savings of 5 minutes, and only 7 percent offered 10-minute savings, they found.
Many carpool lanes accommodate fewer vehicles, going at slower speeds, than they would if they were kept as general-purpose lanes, the researchers asserted. That may seem counterintuitive, but it's because slow carpool drivers block the faster drivers behind them. Kwon and Varaiya found that carpool lanes carry only 1,600 vehicles per hour per lane at an average speed of 45 mph, compared with a capacity of 2,000 vehicles traveling at 60 mph — a 20 percent capacity deficit, the study noted.
Carpool lanes do reduce overall congestion slightly, but only when the regular lanes are congested. And there is no evidence that carpool lanes encourage carpooling, which has been in decline, according to Kwon and Varaiya.
Goodwin said while he might not argue with the general premise of the research, he cautioned that one could only draw those conclusions broadly.
"In individual corridors, that's not true," he said. "The 34-mile stretch of Morgan Hill to the San Mateo County border on northbound Highway 101 is the most heavily used carpool lane in the Bay Area," he said, citing Caltrans 2010 raw data.
During the northbound morning commute, the carpools saved 17 minutes over non-carpoolers along the corridor. That savings is a 43 percent increase over 2009, when it was 12 minutes, Goodwin said.
Officials estimate that a highway lane should be able to carry about 2,000 vehicles an hour. The existing carpool lane on 101 from Highway 85 to Oregon Expressway is currently at 86 percent of its capacity — or 1,730 vehicles — during the peak hour, he said. That's significantly above the average 1,400 for any Bay Area carpool lane during the peak hour, he said.
Still, not all carpool lanes are effective, said Dan Collen, deputy director of infrastructure development at the Santa Clara County Roads and Airports Department. Along county expressways, certain segments of the diamond lane have been removed when studies found the designation hampered traffic flow. Lawrence Expressway north of Highway 101 used to have a carpool lane, but county engineers decided to convert it back to general-purpose use, he said.
"We found it was used but was dominated by non-HOV violators, in part because of the need of users to get in the right lane to Highway 237," he said.
But at San Tomas Expressway, the carpool lanes do well. And on north-south routes, they fill a gap where freeways are absent, and traffic volume is high, he said.
In Santa Clara County, a region that could grow by nearly 513,000 residents and 427,000 jobs between 2010 and 2035, according to the Association of Bay Area Governments, transportation officials must squeeze the maximum capacity out of the existing highway system, Goodwin said.
"The era of freeway construction is behind us. It is going to be about expanding capacity. We're not going to see another Interstate 280," he said.
Adding to the capacity means additional carpool and FasTrak toll lanes for single-occupant vehicles, and other traffic-flow measures, such as metering lights, he said.
By 2020, the stretch of Highway 101 from Santa Clara County to Morgan Hill will have double carpool lanes in both directions, and one of those lanes is expected to be converted to a toll lane, Goodwin said.
A high-occupancy toll lane, or HOT, gives people driving solo access to carpool lanes for a fee. Tolls are collected either by license plate readers or through a transponder that can clock the miles traveled in the lane. The tolls increase and decrease as traffic density and congestion within the tolled lanes changes — known as congestion pricing. Carpools and buses drive in the lane at no charge. Actual rates have not yet been determined, but could range from 14 cents to $1 per mile. On Interstate 680, express lanes currently charge $3 for 14 miles during peak travel time, according to the Metropolitan Transportation Commission.
Kwon and Varaiya recommended strategies such as double carpool lanes. Part of the traffic-flow problem with single carpool lanes is because of so-called "snails" — slower drivers who force faster carpoolers to dart back into the regular lanes during peak commute times to try to pass.
As volume in carpool lanes increases, there are more snails, leading to a drop in speed, they wrote. Double carpool lanes would allow carpoolers to get around slower drivers and still be in the fast lane.
A 4-mile section at state Highway 237 and Interstate 880 opened as a test toll lane on March 20.
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