Carpool lane opens on southbound Highway 101 in Petaluma

The newly added portion lengthen the carpooling lanes to 60 miles between Petaluma and Windsor.|

A three-year, multimillion-dollar highway widening project has reached its final milestone with the opening of the southbound carpool lane on Highway 101 in Petaluma, Caltrans and the Sonoma County Transportation Authority announced in a Tuesday news release.

The Petaluma Widening Project added north and southbound carpool lanes between Corona Road and Lakeville Highway, which fill a gap between existing carpool lanes that extend north to downtown Windsor and south to the Marin-Sonoma border. The newly added portion lengthens the carpooling lanes to 60 miles between Petaluma and Windsor.

“By combining (Senate Bill) 1 funds with county funds, we set this project in motion, and ultimately bridged the gap in the carpool lanes, which will reduce congestion, encourage transit and carpool use, and improve the quality of life for all Californians who travel this corridor,” Caltrans District 4 Director Dina El Tawansy said.

The Petaluma Widening Project included raising and widening the North Petaluma Overhead; building a bridge over the Sonoma-Marin Area Rail Transit tracks and a new bridge over Rainier Avenue; adding a new northbound auxiliary lane between Lakeville Highway and East Washington Street; and installing four new sound walls.

The roadwork, which amounted to $122 million, included $85 million in funding from Senate Bill 1 — the Road Repair and Accountability Act of 2017 — and $37 million in funding from Sonoma County’s Measure M.

It reached a milestone in November 2021, when the northbound carpool lane opened.

The project is one of 13 in the Marin/Sonoma Narrows Widening Project, a decadeslong regional effort to add 17 miles of carpool lanes in each direction on Highway 101 between Highway 37 in Marin County and Corona Road in Petaluma.

Caltrans and the Transportation Authority of Marin also broke ground Thursday on the final segment of new carpool lanes at the Marin-Sonoma County line. That portion of the project costs $135 million, including $44 million in funding from Senate Bill 1, $77 million in federal funding and $13 million in local funds.

SB 1 provides $5 billion in transportation funding annually shared equally between the state and local agencies.

Amelia Parreira is a staff writer for the Argus-Courier. She can be reached at or 707-521-5208.

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