Another piece of the Highway 101 improvement project through the Sonoma-Marin Narrows opened Friday, extending a frontage road south from the line dividing Sonoma and Marin counties, Caltrans announced.

The new frontage road opened to drivers is an extension of San Antonio Road, which runs along the western side of Highway 101 from the Sonoma-Marin county line to the Redwood Landfill interchange.

Direct access to Highway 101 from San Antonio Road is closed, and motorists now need to use the Redwood Landfill interchange to access the freeway.

It is the second new frontage road to open in The Narrows between Petaluma and Novato. Last month, Caltrans opened a frontage road connecting Atherton Avenue in Novato with Olompali State Park, eliminating the direct access to the park from Highway 101.

Work in the eight-mile Narrows is expected to last through 2017. At that time, Highway 101 will have new interchanges, bridges, frontage roads and safety improvements. A separate project to widen the highway to six lanes is planned but faces a $250 million shortfall.

You can reach Staff Writer Matt Brown at 521-5206 or On Twitter @PDRoadWarrior.