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Marin-Sonoma Narrows

Monthly Construction Update (Posted August 6, 2014)

Check out the progress being made on the Marin Sonoma Narrows project

The Narrows on US Highway 101 (US101) is a nationally and regionally significant highway linking the San Francisco Bay Area and Oregon and is the only continuous north/south route through Marin and Sonoma Counties. The State has designated the Narrows as an inter-regional route of significance, and Highway 101 is listed as a Focus Route in California’s 1998 Interregional Transportation Strategic Plan.  As a result of these designations the MSN Project has been the recipient of state discretionary funding on four separate occasions. The MSN Project is also included in the 2010 Regional Transportation Plan and the 2002 Key Transportation Corridors in the Bay Area Region.Marin Sonoma Narrows Photo

The existing US101 through the Narrows has two lanes in each direction, has sections that do not meet current freeway standards and consistently maintains a poor level of service in many sections.  The MSN Project will widen approximately 17 miles of US101 from four to six lanes by adding one high occupancy vehicle (HOV) lane in each direction; create a controlled access freeway section through the historic “Narrows”, and upgrading the highway generally to current freeway standards from Route 37 in Novato (Marin County) to Old Redwood Highway in Petaluma (Sonoma County).  It is part of the overall regional plan to provide continuous HOV lanes through Marin and Sonoma Counties.

Caltrans developed an Environmental Impact Statement/Environmental Impact Report (EIS/EIR) for the entire project.  On August 5, 2009, the Final EIS/EIR document was made available on the Caltrans website. Marin Sonoma Narrows at San Antonio

In May 2008, the California Transportation Commission (CTC) awarded $66.04M in Inter-regional Transportation Improvement Program (ITIP) funding to the project.  In 2012, the CTC increased the Proposition 1B Corridor Mobility Improvement Account (CMIA) funds from $82.4 million to $172.6 million in addition to previously committed federal, state and regional funding.  This increases available funding to nearly $398 million out of an estimated $745 million total project cost.


TAM, the Sonoma County Transportation Authority (SCTA), and Caltrans have developed the following projects which make up the Phase 1 Project. They are:

  • A1 - HOV Lanes in Novato  – Primarily addresses congestion by adding HOV lanes through median widening, which includes NB HOV lanes from Highway 37 to north of Atherton Boulevard and SB HOV lanes from Highway 37 to Rowland Boulevard.  The HOV lanes opened to traffic in Summer 2012.

  • A2 - Extend Southbound HOV Lane  -  to Franklin overhead.  Construction started in June 2013 and is expected to be completed by the end of 2013. 

  • A3 - Extend Northbound HOV Lane -  from Atherton to 1.4 miles south of the Redwood Landfill Interchange. Construction started in April 2013 and is expected to be completed in Spring 2014.

  •  B1 - Southerly Interchange  – constructs a new interchange and supporting frontage roads to serve San Antonio Road and closes uncontrolled access points.  Construction started in April 2013 and is expected to be completed in Fall 2015.

  • B2- Petaluma Boulevard South Interchange  - constructs a new interchange and supporting frontage roads to serve Petaluma Boulevard South and closes uncontrolled access points.  This project also includes construction of the Petaluma River Bridge to accommodate future HOV lanes. Construction started in April 2013. 

  • B3 – San Antonio Curve Correction  – reconstructs the mainline highway curve across the Marin/Sonoma county line at San Antonio Creek to improve sight distances and avoid flooding issues.  A new US101 bridge over San Antonio Creek is included in this Project.  Funding for this Project is scheduled to be available in July 2014.

  • C3 - Replace the Northbound Route 116(E) Separation bridge and widen Southbound separation bridge to accommodate HOV lanes on US101. Construction expected to start in April 2013.

A continuous bike route is also planned through all projects by utilizing a combination of class I and II bike lanes.

Staff Contact:
Jit Pandher, PE
Phone:  (415) 226-0822
Email:   jpandher@tam.ca.gov

Additional information can be found on the Caltrans District 4 project website.

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Last updated: 8/20/2014 9:56:01 AM