District 4 - 101 Marin-Sonoma Narrows B3 San Antonio Curve Correction Project

Construction will begin in Spring 2016 and continue through 2018. The project will realign US Route 101 at San Antonio Road to upgrade the roadway profile and alignment, and replace the US Route 101 San Antonio Creek Bridge. A bicycle facility will be constructed along San Antonio Creek, connecting San Antonio Road constructed under Contract B1 (Redwood Landfill Interchange) with the Petaluma Blvd. South frontage road currently being constructed under Contract B2. Please note that the new MSN frontage roads will not connect through vehicular traffic, but will accommodate bicycles and pedestrians.
  • This construction project will enhance sight distance, improve roadway grades, and upgrade the conventional highway to modern freeway standards by modifying the alignment of US 101 and replacing the San Antonio Creek Bridge at the Marin/Sonoma County line.
  • Approximately 500,000 cubic yards of soil will be excavated from the grade south of the Petaluma City limits and will be placed and compacted in the valley adjacent to San Antonio Creek to “flatten” the roadway.
  • The realigned freeway will be located immediately to the west of the current highway and the existing northbound 101 lanes will become part of the Petaluma Boulevard South frontage road extension.
  • The new roadway and bridge will be constructed to accommodate future high-occupancy vehicle lanes but will be striped as a four-lane facility at completion.
  • Construction on this project will begin in early 2016 and should take approximately 3 years to complete.
  • As part of this project, in late March or April 2016. Please see construction notice for more information.
  • Bicycle Detour: Due to construction activities and staging, bicyclists will not be allowed in the construction zone. During construction, please use San Antonio Road and the D Street Extension
Both the B3 project and the B2 project to the north (Kastania Interchange/Petaluma River Bridge) will convert U.S. Route 101 from an expressway to a freeway. This important safety component will build or extend frontage roads and eliminate driveway connections to U.S. Route 101.
  • On the southbound side of the freeway, Kastania Road will extend to Gunn Drive, and drivers will be able to access it from the new Kastania interchange constructed under Contract B2. Locations further south by San Antonio Road will be accessed from the Redwood Landfill Interchange.
  • By spring or early summer of 2016 the only access to the frontage roads will be via the two interchanges. At that point the expressway to freeway conversion should be mostly complete.
  • On the northbound side of the U.S. Route 101, Petaluma Boulevard South will be extended to the north side of San Antonio Creek. When the roadway is realigned in the B3 phase of the MSN 101 project, old northbound lanes will become part of Petaluma Boulevard South frontage road.
  • The frontage roads will never connect, so they will not be used to bypass the freeway. However, at the San Antonio Creek Bridge, a bike/pedestrian pathway will connect from Petaluma Boulevard South to San Antonio Road.
For more information about the project, please contact:

Alejandro Lopez
Caltrans Public Information Officer, Sonoma County
(510) 286-5445

Steve Williams
Caltrans Public Information Officer, Marin and San Francisco Counties
(510) 286-6195

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