Marin-Sonoma Narrows project costs climb as work goes on

All lanes of a Highway 101 widening project between Marin and Sonoma have been open since late September, but construction isn’t over and costs are creeping up.

Since the final phase of the Marin-Sonoma Narrows project got underway in 2022, inclement weather and unplanned work have challenged the crews, according to the California Department of Transportation. The last phase, called the “MSN B7” project, installed carpool lanes on 3.5 miles between Novato and the Sonoma County line.

With construction cost overruns and change orders, the final phase is expected to increase about $2.5 million to nearly $108 million.

The most significant increase is because of cross-slope correction on the highway where the new pavement meets the asphalt that was installed in an earlier phase. The job will smooth out the drive by grinding the pavement to level the grade, which was misaligned because of unforeseen site conditions. The cost of the unexpected work is estimated at $1.3 million.

“Essentially, the new road is being blended with the existing road,” said Marin County Supervisor Eric Lucan, chair of the Transportation Authority of Marin board. The agency is a partner in the Caltrans project.

“Any cost increase is a cost increase, right?” Lucan said. “When you put this much money into a project, we really appreciate folks in the field doing these checks to make sure it’s going to stand up in the long run, making sure their getting these details right.”

Another factor for the increase is that as winter approached, crews pushed off the final paving for a section of the highway until the spring, officials said. Temporary lane striping was needed in the interim.

Other remaining tasks include electrical work for ramp metering and traffic monitoring infrastructure; drainage improvements; erosion control; the installation of a concrete barrier near San Antonio Road; and the installation of signs.

“Completing the remaining MSN B7 components is a high priority,” said Matt O’Donnell, a Caltrans representative.

“The ramp metering and traffic monitoring systems are expected to help alleviate the operational issues currently affecting the corridor,” O’Donnell said. “In addition, the final asphalt lift, permanent striping and signage are necessary to complete the pavement structural section, accommodate existing traffic volumes and ensure roadway safety.”

Daytime and nighttime closures will be required, officials said. Work is expected to wrap up this spring.

An executive committee of the Transportation Authority of Marin has recommended approving a contract amendment, authorizing an allocation of up to $2.5 million to Caltrans to support the project. The TAM board is expected to approve the amendment at its meeting on Thursday.

“This will probably not be the final amendment,” said Dan Cherrier, a project manager at the Transportation Authority of Marin.

Construction support costs for the project are $17.35 million. Cherrier said he expects that to increase.

“At this time, the project remains within the currently allocated $17.35 million,” O’Donnell said. “However, Caltrans will coordinate with project partners should an increase become necessary.”

In whole, the Marin-Sonoma Narrows widening project was estimated at about $762 million. The actual cost will be determined once agencies close out contracts, officials said.

The corridor gained the “Novato Narrows” nickname years ago because the road contracted to two lanes in each direction, creating daily traffic jams. The widening project added a third lane north and south on the 17-mile stretch between Novato and Petaluma. Now there is a contiguous stretch of carpool lanes for about 50 miles from the Richardson Bay Bridge in Marin to Windsor in Sonoma County.

Another change to the corridor is expected soon: reduced carpool lane hours.

Caltrans had imposed expanded carpool hours in September to coincide with the completion of the Marin-Sonoma Narrows project. However, drivers complained the expanded hours created chokepoints that never existed before.

The carpool hours were expanded to 5 to 10 a.m. and 3 to 7 p.m. on weekdays in both directions. The intent was to make restrictions consistent in both counties and to align with the hours on the state-owned bridges.

Previously, carpool hours in Marin were 6:30 to 8:30 a.m. southbound and 4:30 to 7 p.m. northbound. In Sonoma County, the hours were 7 to 9 a.m. and 3 to 6:30 p.m. in both directions.

Last month, Caltrans, under pressure to implement shorter carpool restrictions, said it came to an agreement with its partners in the two counties and the Metropolitan Transportation Commission to make the change.

The exact hours will be determined after a traffic analysis, due to be completed this month. Once finalized, the new hours for high-occupancy vehicles, or HOVs, could change in February, transportation officials said.