Highway 101 carpool lane opening in Petaluma

A temporary carpool lane is expected to open early Thursday on a stretch of Highway 101 that’s undergoing a widening project in Petaluma.

The new northbound lane is on the Washington Creek Bridge between Lakeville Highway to the south and the Lynch Creek Trail bridge to the north, according to Caltrans.

It was expected to be open by 5 a.m. Thursday, Caltrans spokesman Jeff Weiss said.

“There will be three lanes the whole way, which is the first time it’s happened there,“ he said.

During construction, the left and right lanes will be 12 feet wide and the middle lane will be 11 feet wide. All lanes will be 12 feet wide once the project is done, Weiss said.

The extra lane is expected to relieve congestion and improve traffic flow, since travelers have historically needed to slow down when going from three lanes to two lanes just before the bridge.

“There’s a lot of merging that occurs,” Weiss said.

The carpool lane is in a temporary location while southbound lanes are widened and should move to a permanent area in late 2022 when construction wraps up in the adjacent lanes.

Even in its temporary state, it closes a gap in carpool lanes between the Marin County border and Windsor — a stretch of about 30 miles.

Once completed, the entire width of the freeway will increase from about 48 feet to 120 feet.

It’s only a portion of the much-larger effort to expand Highway 101 from two lanes to three lanes along a 16-miles stretch of Highway 101 from state Route 37 in Marin County to Corona Road in Petaluma.

The Marin-Sonoma Narrows Project has been ongoing for 20 years and the current work is part of its final phase. The entire project is expected to cost at least $754 million, and the existing phase should wrap up by late 2022.

Once the Petaluma portion is done, all that’s left is a 6-mile Marin County segment from Atherton Avenue in Novato to San Antonio Road, south of the Marin-Sonoma county line.

That widening work begins this spring and should be finished in late 2023 or early 2024.

You can reach Staff Writer Colin Atagi at colin.atagi@pressdemocrat.com. On Twitter @colin_atagi.