The California Department of Transportation (Caltrans), the Sonoma County Transportation Authority and the Metropolitan Transportation Commission have commenced construction on the next phase of the Marin-Sonoma Narrows Project (MSN) on US Highway 101 (US 101) in Petaluma, California.

The $700m MSN Project includes the widening of US 101 from Novato to Petaluma, as well as other major improvements.

This segment of the highway is known as The Narrows for its reduction to two lanes, and has experienced heavy congestion in the past.

The MSN programmes in Petaluma were granted $120m in funding, including $105m from Proposition 1B, a 2006 voter-approved transportation bond, and $7m from Measure M, approved by Sonoma County voters in 2004.

In total, close to $15bn in Proposition 1B funds have been distributed across the state.

Caltrans director Malcolm Dougherty said that the latest project is another step in achieving the goal of reducing congestion and increasing safety along US 101.

"The $700m MSN Project includes the widening of US 101 from Novato to Petaluma."

The work in Petaluma includes upgrading US 101 from a highway to a freeway; reconstructing the Petaluma Boulevard South interchange, replacing the Petaluma River Bridge, and constructing or widening about three miles of frontage roads.

The work also includes providing new pedestrian and bicycle enhancements, replacing the northbound State Route 116 Bridge, and widen the southbound SR-116 Bridge to accommodate future carpool lanes on US 101.

Caltrans District 4 director Bijan Sartipi said this work is the foundation for eventual carpool lanes and will bring the highway up to freeway standards.

"When completed, the highway will be wide enough to accommodate a third lane once the carpool lane projects come through," Sartipi said.

The MSN Project proposes to add high occupancy vehicle (HOV) facilities across 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.