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Project Background

Marin-Sonoma Narrows
Project Map

U.S. Route 101 is the principal route in the coastal northwest between the San Francisco Bay Area and Oregon, and the only continuous north/south route through Marin and Sonoma Counties. Over the past 15 years, recreational expansion, commercial, and population growth in Sonoma and Marin counties have led to significant traffic increases along the US 101 corridor.

In 1998, the California Department of Transportation (Caltrans) developed a Statewide System Management Plan which noted that US 101 in Marin and Sonoma Counties could not handle projected traffic growth and that solutions were needed.

One of the projects intended to implement congestion relief along US 101 is the Marin-Sonoma Narrows (MSN) Project. The MSN Project extends 17 miles between State Route 37 in Novato (Marin County) and Old Redwood Highway in Petaluma (Sonoma County) along a rural, largely undeveloped corridor. The project name derives from a narrowing of the existing facility from six to four lanes between north side of Novato and the south side of Petaluma, a segment commonly referred to as “the Narrows.”

The MSN Project proposes to add High Occupancy Vehicle (HOV) facilities throughout the corridor to adjoin the HOV lanes south of the project limits and tie to the proposed HOV lanes to the north extending from Petaluma to Windsor. Widening to include HOV facilities will require replacing the Petaluma River Bridge and upgrading the “Narrows” segment of the corridor from an expressway to a freeway with controlled access points.

Caltrans, the Sonoma–Marin Area Rail Transit (SMART) project, local agencies, and transit providers continue to coordinate to ensure compatibility between MSN roadway improvements and mass transit facilities and to provide for continuous bicycle and pedestrian paths throughout the corridor.

The MSN project area is divided into three segments: A, B, and C (see map). Although these segments were originally presented as three separate projects, they have been combined into a single study area to ensure that any impacts are addressed cumulatively in a comprehensive Environmental Impact Study/Environmental Impact Report (EIS/EIR). Once this EIS/EIR process has been completed and a preferred alternative has been identified, funding will be sought for constructing individual project elements based on the operational priority (level of need) and funding availability. Caltrans and its partner agencies, the Transportation Authority of Marin and Sonoma County Transportation Authority, continue to seek new funding sources for the project.


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