The Marin-Sonoma Narrows Project

The Marin Sonoma Narrow extends 16-miles from State Route 37 in Marin County to Corona Road in Petaluma. The project has been divided into 13 sub-projects to correspond with funding availability. Eleven projects have been completed; two are under construction, and one will begin the future when funding becomes available.

The ABCs of the Project

The projects were assigned names (Segment A, B or C) according to the location of the project. The map below will help to identify the projects.

  • Segment A projects - Novato from south of State Route 37 to Atherton Avenue.
  • Segment B projects - Novato from Atherton Avenue to Route 116 in Petaluma
  • Segment C projects - Petaluma from State Route 116 to the Corona Road

What Projects are Left for the Marin/Sonoma Narrows Widening?

Of 13 projects, four remain to be completed.  One project, C2, the Petaluma Widening, is currently under construction, and should be completed by late 2024. 

Of the three projects that have not started, the biggest one is project B7, which will widen Highway 101 between Novato and the Marin/ Sonoma county line. 

The final widening project, B7, will occur between Novato and the Sonoma/Marin county line. The project has been designed, but construction was postponed by litigation. Some funding will come from Regional Measure 3 (RM3), money derived from bridge tolls. The Howard Jarvis Taxpayers Association sued, alleging that toll money must be used on toll bridge projects. However, in May 2021, the Metropolitan Transportation Commission loaned the project the necessary funding to begin construction.  Caltrans can now put the project out to bid.  

Expect construction to beg in Spring 2022 and be completed in late 2024. Upon completion, all of Highway 101 widening will be finished. 

Two small project needs to be completed, B8 and B3.  

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

B3 is a small project to renovate the San Antonio Creek Bridge to prevent it from flooding. This bridge restoration project is located on a nearby county road and will not affect the freeway widening. 


  • Marin Sonoma Narrows Map

Projects Currently Under Construction Now

  • MSN C2: Construct northbound and southbound HOV lanes through Petaluma, replace Petaluma Bridge, and build sound walls in Petaluma.

Completed Projects

  • MSN A1, Fall 2011 - Winter 2012: Added northbound HOV lane from SR 37 to Atherton Ave (3.2 miles). Added southbound HOV lane from SR 37 to Novato Creek (1.6 miles).
  • MSN A2, June 2013 - Winter 2013: Extended southbound HOV lane from Novato Creek to Franklin Overhead.
  • MSN A3, April 2013 - Fall 2014: Extend northbound HOV lane from Atherton to 1.4 miles south of Redwood Landfill Interchange.
  • MSN B1, April 2013 - Spring 2016: Modify Redwood Landfill interchange and frontage roads and provide new pedestrian and bicycle facility.
  • MSN B2, April 2013- Winter 2016: Modify Petaluma Blvd South interchange and frontage roads, replace Petaluma River Bridge.
  • MSN B3, Spring 2016 - Winter 2019: Realign US 101 at San Antonio Road to upgrade roadway profile and alignment.
  • MSN C1, 2011 - Spring 2014: Construct new East Washington northbound onramp and modify interchange.
  • MSN C3, Spring 2013 - Winter 2015: Replace the northbound U.S 101 bridge and widen southbound bridge over State Route 116 to accommodate future HOV lanes in both directions. Re-construct the northbound on-ramp/off-ramp and sound wall along the northbound on-ramp to U.S. 101.
  • MSN B2 Phase II: Construct northbound and southbound HOV lanes between Petaluma Boulevard South and the Marin/Sonoma County line and stripe for HOV lanes from Petaluma Boulevard South to Route 116.