Caltrans has decided on new carpool lane restrictions on Highway 101 between Mill Valley and Windsor, but Marin planners worry the change could create traffic chokepoints in the county.

Last week, the board governing the Transportation Authority of Marin voted unanimously to send a letter to Caltrans accepting the new commuter lane hours of operation. However, the letter states that a recent analysis shows the new restrictions “would introduce additional local auto and transit impacts, by increasing queue length and duration of congestion” during the morning commute in San Rafael and in the afternoon commute in Corte Madera.

“TAM remains concerned,” the letter says, that the “regional hours will result in additional delay to HOV and general purpose lane users, as well as to transit services that provide local service on the corridor within the general purpose lanes.”

A regional committee has been under pressure to sort out the conflicting commuter lane restrictions before the Marin-Sonoma Narrows widening project is fully operational this summer. 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.

Once all lanes are open, a contiguous stretch of carpool lanes will run for about 52 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, planners said.

Carpool hours on 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.

This month, the committee recommended that Caltrans align the commuter hours on this stretch with the same hours established on the seven state-owned bridges in the Bay Area, including the Richmond-San Rafael Bridge. Caltrans agreed.

The new carpool hours will be from 5 to 10 a.m. and 3 to 7 p.m. on weekdays in both directions.

Derek McGill, director of planning at the Transportation Authority of Marin, said the completion of the widening project will provide an overall benefit to drivers in commuter lanes and general purpose lanes.

A study showed that with all lanes open and carpool restrictions in place, there would be logjams at North San Pedro Road in San Rafael during the southbound morning commute and at Tamalpais Drive in Corte Madera during the northbound afternoon trek.

As a near-term solution, planners at the Transportation Authority of Marin, the Metropolitan Transportation Commission and Caltrans are working to begin ramp metering at four onramps by June 24. They include the northbound onramp at Gate 6 in Sausalito, the two northbound onramps at the State Route 131 interchange in Mill Valley and the northbound onramp at Tamalpais Drive in Corte Madera.

TAM staff said the metering at these ramps would help alleviate back up concerns.

Regional planners also have another project teed up to activate ramp meters in 2026 between Spencer Avenue in Sausalito and Sir Francis Drake Boulevard in Larkspur.

Kevin Chen, an official with the Metropolitan Transportation Commission, said his agency is searching for funding to upgrade all the meters in Marin with new technology. The “adaptive” ramp meters would allow the signal management system to “see” traffic conditions on the whole corridor, Chen said.

“That way we can control the metering rates a little bit more effectively,” Chen said.

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.

In its letter, TAM says its officials are pleased with Caltrans’ commitment to the metering project and that they are also interested in adding adaptive meters at other southbound areas in Novato. TAM said also said it appreciates Caltrans’ commitment to monitor the travel conditions in the corridor and adjust the HOV hours if needed.

Marin County Supervisor Eric Lucan, chair of the Transportation Authority of Marin, said none of the state-owned bridges is contiguous with this section of Highway 101.

The Golden Gate Bridge, which is not owned by the state, operates its carpool lane hours from 5 to 9 a.m. and 4 to 6 p.m. in the southbound direction only.

“I think that’s important,” Lucan said, adding that he “failed to see” why it was recommended the hours align with the state-owned bridges.

“A one-size-fits-all approach sometimes is not always the best approach,” Lucan said.