Marin-Sonoma Narrows highway construction hits new milestone

Drivers heading into Marin can expect to see a new look on Highway 101 after Caltrans completes a milestone in its road widening project.

Beginning Saturday, the state transportation agency will divert southbound traffic onto new elevated lanes spanning from San Antonio Creek to just north of Atherton Avenue in Novato.

The new lanes were constructed as part of the final phase of a decade-long widening project in Marin and Sonoma counties meant to relieve longstanding traffic congestion on the corridor.

The estimated $762 million project will add a carpool lane in each direction along 17 miles of Highway 101 between Novato and Petaluma. The section received its "Narrows" nickname because the road contracts from three lanes to two lanes in each direction.

Once completed, the Marin-Sonoma Narrows project will provide drivers with continuous access to carpool lanes from north of the Golden Gate Bridge to Santa Rosa.

Caltrans has completed all but the 6-mile span from the county line into Novato. The final phase is estimated to cost $135 million.

People who regularly commute into Marin or San Francisco have already seen the new southbound lanes under construction for months within the highway median. The new lanes are 10 feet higher than the existing highway alignment, which Caltrans spokesman Pedro Quintana said will result in an improved line of sight for drivers and better drainage.

"Our crews and contractors are working tirelessly to get this stage done," Quintana said.

After the switch on Saturday, southbound drivers will still have access to two lanes as they do currently, Quintana said.

In preparation for the switch, Caltrans is performing nightly work from 8 p.m. to 4 a.m. this week, which can result in up to two lanes being closed, Quintana said. On Thursday, the agency will close off the southbound onramp and offramp at San Antonio Road until Saturday.

"We're asking people to slow down in the construction zone, so that way crews can be out there, be safe and get the job done," Quintana said.

Nick Nguyen of the Transportation Authority of Marin — the state-managed traffic agency for the county that is a partner on the project — said the new lanes will allow Caltrans crews to shuffle traffic as needed in order to complete the project.

Beginning Monday, construction crews plan to begin excavating and rebuilding the old southbound lanes at the higher elevation, which Nguyen said will likely take about a year to complete.

Once those upgrades are finished, southbound traffic will be moved back onto those lanes. Caltrans will then divert the northbound traffic onto the elevated section in the median in order to begin widening the northbound corridor. When the northbound section is completed, Caltrans will execute the final touches and restripe the highway.

"It's our orchestra that we call freeway construction," Nguyen said.

The northbound construction will take less time to complete because only 3.5 miles of carpool lanes need to be added, compared to 6 miles in the southbound direction, Quintana said.

While Caltrans is aiming to open the new carpool lanes by summer of 2025, Nguyen said the storms from earlier this year could push the completion date off by a few months.

In addition to widening the highway, the project will also create a continuous bikeway between Novato and Petaluma; build new frontage roads to remove unsafe local roads that turned out directly onto the highway; raise sections of the highway out of the floodplain; and add new interchanges.

"We know there is still a lot more work to do but this is a big milestone in the project," said Anne Richman, executive director of the Transportation Authority of Marin.