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Marin Sonoma Narrows

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

  • A3: April 2013 – Fall 2014: Extend northbound HOV lane from Atherton to 1.4 miles south of Redwood Landfill Interchange.
  • B1: April 2013 – Fall 2014: Modify Redwood Landfill interchange and frontage roads and provide new pedestrian and bicycle facility.
  • B2: April 2013 – late 2016: Modify Petaluma Blvd South interchange and frontage roads, replace Petaluma River Bridge. Provide new pedestrian and bicycle facility.
  • C1: 2011 – Spring 2014: Construct new East Washington northbound onramp and modify interchange.
  • C3: Spring 2013 – mid 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.
Completed Projects

  • 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).
  • A2: June 2013 – Winter 2013: Extended southbound HOV lane from Novato Creek to Franklin Overhead.
Future Projects (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.
  • B3: Fall 2014 – 2017: Realign US 101 at San Antonio Road to correct horizontal alignment and construct a new mainline San Antonio Creek Bridge. A bicycle facility will be constructed along San Antonio Creek, connecting frontage roads constructed in Contracts B1 and B2.
  • B5: Funding anticipated after 2018. Reconstruct San Antonio Road to include bike lanes.
  • B6: Spring 2017 – Spring 2018: Replace San Antonio Creek Road (County) Bridge.
  • C2: Construction funding expected after 2018: Construct HOV lanes in both directions from just north of Route 116 to Corona Road.
  • Landscaping and on-site mitigation (tree planting) for projects in construction is expected to begin in 2015 and 2016.