Another incremental piece of the Sonoma-Marin Narrows Highway 101 project opened today, Caltrans said.

Drivers can now use the new frontage road along the western flank of Highway 101 from the Sonoma-Marin county line to the Redwood Landfill interchange. The new road is an extension of San Antonio Road. Direct access to Highway 101 from San Antonio Road is now closed off. Motorists on the road 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, a frontage road opened connecting Atherton Avenue with Olompali State Park, eliminating the direct access to the park from Highway 101.

The two frontage roads do not connect, meaning commuters will still need to use Highway 101 to travel between Novato and Petaluma through The Narrows. A bike path, however, will connect the two frontage roads, and it will be possible to bike from Petaluma to Novato mostly along the Highway 101 corridor. Cyclists will need to use I Street in Petaluma to reach San Antonio Road, where they can link up with the frontage road/bike path system to Novato.

The bike path should be open in October, Caltrans said.

When the project is complete, new frontage roads and bike paths on the Sonoma County side will link to those currently open in Marin and allow riders to follow Highway 101 the entire 8-mile stretch from Petaluma to Novato. Drivers, though, will still need to use the highway as the roads will not connect.