Marin Sonoma Narrows

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The Marin-Sonoma Narrows project will expand the US 101 corridor to six lanes throughout the Narrows, adding High Occupancy Vehicle (HOV) facilities, and updating infrastructure connecting the freeway to local roads. The current construction started in 2011 and is scheduled to be completed in 2016, with future phases to continue as funding becomes available.

Transportation partners for the MSN 101 project include Caltrans, the Federal Highway Administration, Sonoma County Transportation Agency, Transportation Authority of Marin, Metropolitan Transportation Commission, Marin and Sonoma counties, and the cities of Novato and Petaluma.

Projects on this list are divided up by location.

  • Segment A projects are in the City of Novato from South of State Route 37 to north of Atherton Avenue.
  • Segment B projects begin in the City of Novato and end in the City of Petaluma, beginning north of Atherton Avenue and continuing to State Route 116.
  • Segment C projects are in the City of Petaluma from State Route 116 to the Corona Road overcrossing.

  • Projects Currently Under Construction

  • MSN B2, April 2013 – late 2016: Modify Petaluma Blvd South interchange and frontage roads, replace Petaluma River Bridge. Project will be completed by the end of 2016.
  • The Petaluma Blvd South Interchange project will construct a new interchange at Petaluma Blvd South at Kastania Road, construct frontage roads and replace the Petaluma River Bridge. The Petaluma River Bridge was an 866 feet long, twin reinforced concrete box girder (with pre-cast girder span over the river) bridge that was built in 1955. It had two lanes of traffic in each direction and non-standard shoulders. The new bridge will be 907 feet long and will accommodate three lanes of traffic in each direction with standard shoulders. This will be one of the longest precast, post-tensioned spliced concrete girder bridges in the U.S. Constructing the new bridge over the Petaluma River, which is a navigation channel, has been very challenging due to environmental restrictions, coordination with the U.S. Coast Guard and mariners, and staged construction work to maintain two lanes of travel in each direction of U.S. 101 during construction. The new bridge requires erection of 99 girders up to 130 feet in length and weighing up to 120,000 pounds each.
  • The project will also replace the existing South Petaluma Blvd On/Off Ramps constructed underneath Highway 101 in the mid 1950's with a new "diamond" interchange with a decorative gateway structure at the Kastania Road Overcrossing. The cost of this project is $135 million and is being funded by the California Proposition 1B Transportation Bond Program and Sonoma County Measure M Traffic Relief Act.
  • Upcoming activities include constructing the third and final portion on the southbound side of the bridge. Construction crews are also working on new frontage roads that will upgrade the conventional highway to modern freeway standards by removing direct driveway access to U.S.101.
  • It is important to note that U.S. 101 will remain striped as a four-lane facility following the completion of this project in late 2016. This project is foundational for the ultimate widening of U.S.101 in northern Marin and Southern Sonoma Counties. Future highway widening construction projects are being planned that will add high-occupancy vehicle lanes to the north and south of the new Petaluma River Bridge.
  • MSN C3, Spring 2013 – late 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. Project will be completed in late 2015.
  • The Route 116 (Lakeville) interchange project will replace the northbound U.S 101/116 separation bridge and widen southbound U.S 101/116 separation bridge to accommodate future HOV lanes in both directions from the Petaluma River Bridge to State Route 116 with standard shoulders. The northbound U.S. 101 on-and off-ramps at State Route 116 will be realigned to enhance sight distance and the facilities will be updated with standard features. Sound walls have been constructed along the on-ramp from S.R. 116 to northbound U.S. 101.

  • 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 – Fall 2014: Modify Redwood Landfill interchange and frontage roads and provide new pedestrian and bicycle facility.
  • MSN C1, 2011 – Spring 2014: Construct new East Washington northbound onramp and modify interchange.


  • Future Projects, Funded (All Timelines are Tentative)

  • MSN B3, Spring 2016 – 2018: Realign US 101 at San Antonio Road to upgrade roadway profile and alignment. Construct a new mainline US 101 San Antonio Creek Bridge. A bicycle facility will be constructed along San Antonio Creek, connecting San Antonio frontage road constructed on Contract B1 with Petaluma Blvd. South frontage road constructed on Contract B2. Please note that the new MSN frontage roads will not connect through vehicular traffic, but will accommodate bicycles and pedestrians.


  • Future Projects, Unfunded (All Timelines are Tentative)

  • B1 Phase II: Funding anticipated after 2018: Construct northbound and southbound HOV lanes between the Marin County/Sonoma County line and Atherton Avenue
  • B2 Phase II: Construction funding anticipated by 2018. 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.

Marin Sonoma Narows Financial Plans (Download 87mb Zip file)

Statewide Alerts and Other Information

Marin Sonoma Narrows

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