Avg. daily car traffic


Avg. daily truck traffic


Project Length in Miles


Funds Allocated

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marin sonoma narrows

The Narrows on US Highway 101 (US101) links the San Francisco Bay Area and Oregon and is the only continuous north-south route through Marin and Sonoma counties. Issues with the route include an insufficient number of lanes through the Narrows, sections that do not meet current freeway standards and a consistent poor level of service (e.g., delays, slow traffic, traffic jams) in many sections. This project is included in the 2010 Regional Transportation Plan and the 2002 Key Transportation Corridors in the Bay Area Region.

Project objectives:

Widen 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 (no traffic signals or intersections) through the historic “Narrows” to improve traffic flow

Upgrade the highway to current freeway standards from Route 37 in Novato (Marin County) to Old Redwood Highway in Petaluma (Sonoma County)

In addition to the carpool lanes, new interchanges and frontage roads are being built to remove unsafe access from private properties and local roads. The project also includes continuous Class I and Class II bikeways between Novato and Petaluma.

For more details, visit the Caltrans website

Current Projects

This project is in Phase 1, which includes the following sub-projects.

Expected Completion
2016 Petaluma Boulevard South Interchange (B2): Constructs a new interchange and supporting frontage roads to serve Petaluma Boulevard South and closes uncontrolled access points. Includes construction of the Petaluma River Bridge to accommodate future HOV lanes.
Late 2018/Early 2019 San Antonio Bridge Replacement & Curve Improvement (B3): Realigns the highway westward in the vicinity of San Antonio Creek, establishes a new frontage road for improved access, and raises the grade of the bridge at the creek to address periodic flooding concerns.
Various Bike lanes: A continuous bike route is planned from Novato to Petaluma, using a combination of Class I and Class II bike lanes.
2016 US101 Planting and Orange Avenue Soundwall (L1A): Planted trees and other vegetation along the constructed corridor in Novato. Installed a soundwall on the northbound lanes between DeLong Avenue and Atherton Avenue

Completed Projects

Completion Date
2015 Northbound Route 116(E) (C3): Replaced separation bridge and widened southbound separation bridge to accommodate HOV lanes on US101.
2014 Southerly Interchange (B1): Constructed a new interchange and frontage roads to serve San Antonio Road, and closed uncontrolled access points. Included Class I and Class II bike paths.
2014 Northbound HOV Lane (A3): Extended the northbound HOV lane from Atherton to 1.4 miles south of the Redwood Landfill Interchange.
2013 Southbound HOV Lane (A2): Extended southbound HOV lane to Franklin overhead.
2012 HOV Lanes in Novato (A1): Addressed congestion by adding HOV lanes through median widening, which included northbound HOV lanes from Highway 37 to north of Atherton Boulevard and southbound HOV lanes from Highway 37 to Rowland Boulevard.

Future Phase II Projects, Unfunded (all timelines tentative)

Anticipated Dates
After 2018 for Funding B1 Phase II: Construct northbound and southbound HOV lanes between the Marin County/Sonoma County line and Atherton Avenue
By 2018 for Funding 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.
C2 and B2 Phase II: Add northbound and southbound HOV Lanes on US101 in Petaluma (Sonoma County).
B1 Phase II: Add northbound and southbound HOV Lanes on Hwy 101 in northern Marin County.
A4 Phase II: Add a southbound HOV lane through Novato.
B5 Phase II: Add northbound and southbound HOV lanes, frontage road improvements and a bike path between the Sonoma and Marin county line.

Available Funding

Available funding for Phase 1 is nearly $398 million out of an estimated $745 million total project cost:

2008 $66.04 million from the California Transportation Commission (CTC) Inter-regional Transportation Improvement Program (ITIP)
2012 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.

Bird's Eye View of Marin Narrows

San Antonio Creek Area Construction

Fast Forward Drone