Project Spotlight: Marin-Sonoma Narrows

16-mile stretch of U.S. Highway 101 is getting a makeover that brings HOV lanes, enhanced safety

In baseball terms, Caltrans District 4’s $135 million Marin-Sonoma Narrows Project is in the later innings, but there’s still plenty of game left.

The project’s final component, categorized as B7 among 13 sub-projects, began in July 2022. During construction, U.S. Highway 101 will be renovated between northern Novato and the Sonoma County/Marin County line.

The sub-projects were assigned letter-number combinations based on their location. Segment A work, completed between 2011 and 2014, was done in Novato from south of State Route 37 to Atherton Avenue. Segment B work, begun in 2013, has been from Atherton Avenue in Novato to State Route 116 in Petaluma. Segment C sub-projects, done between 2011 and 2022, were in Petaluma from State Route 116 to Corona Road.

Ongoing sub-project B7 will complete and allow continuous HOV lanes on the roadway from Sausalito in Marin County to Windsor in Sonoma County. HOV lanes are scheduled to be open to use in the winter of 2025-26.

The Marin-Sonoma Narrows Project, which began in the autumn of 2011, is on track for full completion – including landscaping establishment – in winter 2026-27. It includes $43.6 million in funding from Senate Bill 1, The Road Repair and Accountability Act of 2017.

In addition to what is being done for sub-project B7, two smaller components need to be completed:

B6 will renovate the San Antonio Creek Bridge to prevent it from flooding. This bridge is located on a nearby county road and will not affect the freeway widening.

B8 is a utility relocation project that covers that same stretch of highway as B7. Typically, utility relocation must be done before the work begins, but in this case the utilities are not in the work area and can be moved after the B7 project is completed.

Construction activity during B7 on U.S. Highway 101 includes installing HOV lanes for 3.5 miles along the northbound lanes from North Novato to the Marin, Sonoma line, and for 6.3 miles along the southbound lanes from De Long Avenue to the county line.

On June 10, 2023, Caltrans diverted southbound Highway101 traffic onto new elevated lanes spanning San Antonio Creek to just north of Atherton Avenue in Novato. This was a major milestone for the project.

Since then, crews have worked to reconstruct approximately four miles of the old southbound lanes. The contractor has repurposed roadway materials of the old Highway 101 to incorporate as base material for the new highway, saving time, money and environmental resources.

To improve sight distance, the new road is as much as 10 feet higher than the old Highway 101 in some areas. Construction is ongoing in this stage to construct seven new retaining walls, widen the existing Franklin Avenue overhead bridge, and complete the approximately 93,000 cubic yards of earthwork in this stage.

A few challenges have been working through weather-related impacts, both this winter and last year. Including the slide and road closure of Redwood Boulevard in Novato parallel to the project, in which the same contractor and construction team had to address not only the reopening of Redwood but also structural changes to the project’s retaining walls in the area. Another obstacle overcome was the continued work through a high water-table during the roadway reconstruction.

Marin-Sonoma Narrows Project portions that have been completed include a modification of the Redwood Landfill interchange and frontage roads, plus the installment of a new pedestrian and bicycle facility (finished in spring 2016); modification of the Petaluma Boulevard South interchange and frontage roads, plus replacement of the Petaluma River Bridge (winter 2016); and construction of northbound and southbound HOV lanes through Petaluma, plus replacement of thee Petaluma Bridge and installation of sound walls.

Learn more about the Marin-Sonoma Narrows Project on this Caltrans webpage.

Source: Matt O'Donnell, District 4 public information officer