Drivers on Tiburon Boulevard should expect weekday delays through fall 2026 as Caltrans crews rebuild curb ramps and sidewalks along State Route 131.

The work began the week of Monday, Sept. 21, according to a city of Mill Valley notice posted Thursday, Sept. 24. The work stretches from about 1,000 feet west of the US-101 interchange to the Tiburon Boulevard/Main Street intersection, according to Caltrans. Crews are starting near the US-101 interchange and working east toward the Tiburon waterfront.

Intermittent one-way traffic control may be in place between 7 a.m. and 5 p.m. Monday through Friday, according to a Caltrans traffic advisory.

Twenty-eight curb ramps are scheduled for phase one.

Some bus stops may be temporarily affected, and pedestrian detours may be necessary at certain stages. Caltrans is coordinating with Marin Transit and the local school district to minimize disruptions to bus routes, the city notice said. The agency did not specify which school district is involved.

The curb-ramp and sidewalk phase is part of the larger SR-131 Capital Preventive Maintenance Project. According to the Caltrans project page, the project spans three segments with new bikeways, pedestrian refuge islands, sidewalk modifications and intersection changes.

Paving is scheduled for spring 2027, during overnight hours from 11 p.m. to 6 a.m., according to a Chamber of Commerce notice. The full project is expected to wrap up by winter 2027.

Tiburon/East Blithedale interchange study moves forward

Separately, the Transportation Authority of Marin (TAM) is advancing a long-term study of the Tiburon Boulevard/East Blithedale Avenue interchange at Highway 101. TAM is selecting consultants to conduct environmental and preliminary engineering studies, the city notice said.

The interchange was one of three that TAM prioritized out of a study of 12 Highway 101 interchanges, according to the city notice and Mill Valley Briefing; TAM's own project page confirms three interchanges were prioritized but does not name them. TAM completed a Project Initiation Document in December 2025 and held workshops to narrow design options in early 2026. Environmental studies are set to begin in late 2026. A final environmental document is targeted for late 2029, with design of a preferred alternative starting around 2029 or 2030.

Construction is not expected to begin before 2033, the city notice said. Construction funding has not been secured, according to TAM project documentation, as Mill Valley Briefing reported in May.

Mill Valley Councilmember Urban Carmel, the city's TAM liaison, said in that May report that he believes construction could begin within five years if all goes well. Planning is funded through Measure AA, the half-cent Marin transportation sales tax approved by voters in 2018. TAM's expenditure plan allocates 3% of Measure AA revenue to the interchange program, estimated at $25 million over 30 years.

The congestion is well documented. Afternoon peak travel times heading east on East Blithedale toward Highway 101 worsened by 4.3% even after the city activated an adaptive signal system, Mill Valley Briefing reported. The two signals closest to the Highway 101 interchange off-ramps are controlled by Caltrans, not the city.

The Caltrans public information officer for the SR-131 project is Matt O'Donnell, reachable at [email protected] or (510) 852-5113.