Marin-Sonoma carpool lane restrictions challenge planners

Planners are under pressure to sort out conflicting commuter lane restrictions as the Marin-Sonoma Narrows widening project on Highway 101 nears an end.

Once all lanes open, a contiguous stretch of carpool lanes will run for about 50 miles from the Richardson Bay Bridge in Marin to Windsor in Sonoma County. But the carpool lane hours in the two counties are different, and that needs to change to create consistent travel, said Connie Fremier, project manager for the Transportation Authority of Marin.

A Transportation Authority of Marin executive committee that oversees administration, planning and projects was briefed on the problem at a meeting on March 10.

“With the opening of the Marin-Sonoma Narrows this summer, we really need a decision on this no later than April,” Fremier said.

Results of a traffic analysis show that the proposed carpool hours of 5 to 10 a.m. and 3 to 7 p.m. on weekdays could cause some chokepoints around offramps and onramps in Marin. Planners are considering whether to adjust the hours or introduce other improvements to help relieve backups.

The 17-mile stretch between Novato and Petaluma received its “narrows” nickname because the road contracted from three lanes to two lanes in each direction, creating daily gridlock. The 13-year, $762 million construction project adds a carpool lane in each direction.

Carpool hours on other parts of Highway 101 in Marin are 6:30 to 8:30 a.m. southbound and 4:30 to 7 p.m. northbound. In Sonoma County, the hours are 7 to 9 a.m. and 3 to 6:30 p.m. in both directions.

“We see the congestion in the corridor really starting around 6:30 a.m., and all the way up to about 9:30 to 9:45 a.m.,” said Derek McGill, director of planning at the Transportation Authority of Marin.

The good news, he said, is that the completion of the widening project will provide an overall benefit to drivers in commuter lanes and general purpose lanes.

However, with all lanes open and carpool restrictions in place, an analysis showed that the northbound Tamalpais Drive exit in Corte Madera is a concern, he said. In the morning southbound commute, there is a trouble point around North San Pedro Road in San Rafael.

In both instances, traffic to get off the freeway gets jammed, he said.

Anne Richman, executive director of the Transportation Authority of Marin, said the northbound afternoon commute is most concerning because models show 2 miles or more of queuing.

“That’s a lot, and that catches the carpools and the buses, too,” Richman said. She said supporting the carpools and buses is an important goal for planners.

Kevin Chen, an official with the Metropolitan Transportation Commission, said planners are considering options such as ramp metering, incident management and coordinated traffic signals.

Ramp metering can be done in a matter of months, because infrastructure is already installed on many ramps, Chen said.

“The key here is that we’re going to continue to monitor the performance of the corridor,” Chen said. “We’re definitely not going to set it and forget it.”

Part of the strategy would also involve public communication so drivers are aware of the coming changes, and enforcement, planners said.

The Transportation Authority of Marin is also working toward a part-time transit-only lane to bypass traffic when speeds slow to 35 miles per hour or less. The agency is expected to select a contractor in the coming months to perform engineering and environmental review for that effort.

Marin County Supervisor Eric Lucan, the chair of the agency’s executive committee, suggested modifying the hours.

“We need to have the flexibility to at least put forward some thoughts and recommendations,” Lucan said.

Chen said that with afternoon traffic building as early as 2:30 p.m., the commuter lane restrictions have to start earlier than the 4 p.m. start time they do now.

“It’s very important for us to keep in mind that we need to have that HOV lane for our carpool and transit users early on in the congestion period,” Chen said.

The comments are expected to be presented to the multi-agency leadership team studying the issue. The participants include members of the Transportation Authority of Marin, the Sonoma County Transportation Authority, the Metropolitan Transportation Commission, Caltrans, the California Highway Patrol, Marin Transit and Golden Gate Transit.

The goal is to build consensus on hours of operation. Caltrans will have final approval.