A realigned, 2.5-mile segment of Highway 101 north of San Francisco opened this month, a major step in widening 16 miles of the heavily traveled Marin-Sonoma Narrows. The larger $743-million project is designed to clear chronic bottlenecks and elevate the roadway from the flood-prone wetlands.

To deliver the project, the Marin and Sonoma transportation authorities, along with Caltrans, divided the job into 18 different contracts, says Marin project manager Nick Nguyen. Four key contracts remain to finish the project by adding carpool lanes to two more segments.

“The Narrows is a major project to solve the congestion issue in this region,” Nguyen says. “This highway would close about once a year due to flooding, depending on storm events, and it’s become much more stormy.”

More than 150,000 vehicles travel the Narrows daily. First planned in the late 1990s, the project gained environmental approval in 2009 and began in 2011-2012, Nguyen says. The highway was moved about 190 ft to the west to soften the highway’s curve and to get the roadway out of the floodplain. The project also upgraded the highway for a 70-mph design speed.

The realignment and widening of Highway 101 required about 500,000 cu yds of cut/fill with native material, says Michael Ghilotti, president of contractor Ghilotti Bros. Inc. To complete the San Antonio Creek bridge, crews had to work around seasonal environmental restrictions, as well as working within a seasonal flood plain around the creek. The design of the bridge required 100% cast-in-place methods, with multiple extensive concrete pours to ensure monolithic sections. 

The Marin transportation authority is pursuing the $105 million needed to add its three final miles of carpool lanes, now in the preliminary design stage, Nguyen says. The money could come from a state gas tax increase and possible bridge toll hike. Sonoma County plans to fund its remaining carpool lanes through local sales tax measures and pending funds from the gas tax increase.

The centerpiece of the newly finished 2.5-mile Narrows segment at the Marin-Sonoma county line is the 510-ft-long bridge over San Antonio Creek, designed to withstand a 7.0-magnitude earthquake and a 100-year flood. It replaces two parallel, 120-ft-long bridges with limited openings for the flood-prone creek to weave through. The new bridge was built about 5 ft higher and won’t restrict the creek’s path, says Randip Bains, senior bridge engineer at Caltrans.