In the San Francisco Bay area — like most suburbs outside major cities — traffic congestion is a regular way of life. The California Department of Transportation (Caltrans) and general contractor, Ghilotti Construction Company, are working on a project to ease congestion along a major route in the San Francisco, California, suburbs.
U.S. 101 runs from Los Angeles, California, to Tumwater, Washington, at the Canadian border. The route also runs through San Francisco and its suburbs.
A 16-mile stretch of U.S. 101 from State Route 37 in Marin County to Corona Road in Petaluma, California, is known as Marin-Sonoma Narrows (MSN). The “Narrows” part of the name comes from a nickname the road received due to the lanes contracting from three to two in the area.
The chokepoint is a source of frustration for the many who regularly use the route. Caltrans has been focused on the area for over a decade as they try to ease its congestion.
The Marin-Sonoma Narrows Project covers the entire 16-mile stretch. According to the Caltrans website, the project was divided into 13 sub-projects beginning in the fall of 2011. Caltrans divided the sub-projects into three segments (A, B, and C).
"The MSN's project plans, specifications, and estimates and construction activities are being performed incrementally using smaller, independent, individual contracts/projects to best utilize cash flow and define scopes of work that make each project operationally independent and provide the most benefit to U.S. Highway 101 corridor's mobility and safety," said Matt O'Donnell, a Public Information Officer in Marin County for Caltrans.
The team is currently in the middle of MSN B7, the final project of the 13. When the B7 project was designed in 2019, the average daily traffic in the area was 164,000.
The northbound and southbound high-occupancy vehicle (HOV) lanes will have multiple benefits. According to O'Donnell, the lanes will:
- Reduce congestion and improve mobility while incentivizing commuters to use buses, carpools, or vanpools for peak-period travel
- Improve freeway operations, including providing safe access to and from the facility
- Improve the safety of the route by increasing visibility, improving drainage to address recurring flooding, and providing wider shoulders
- Construction of 12 retaining walls
- Installation/replacement of dozens of underground culvert replacements
- Widening of two bridge structures
- Congestion Mitigation and Air Quality Program - 59.86 percent
- Senate Bill 1 - Solutions for Congested Corridors Program - 24.70 percent
- Local Highway Infrastructure Program - 12.92 percent
- COVID RIP - 1.11 percent
- Local funds from the Transportation Authority of Marin - 1.41 percent
- Owner: California Department of Transportation (Caltrans)
- General Contractor: Ghilotti Construction Company, Santa Rosa, California
- Designer: BKF Engineers, Oakland, California; Project Engineer, Natalina Vaccaro Bernardi
- Other Key Contractors: Dutra Materials, A CRH Company, San Rafael, California
The last sub-project focuses on a 3.5-mile section of the roadway between North Novato, California, and the Marin-Sonoma county line. The team will also install another approximately 3 miles of HOV lane in the southbound direction within Novato (approximately 25 miles north of San Francisco).
The team is raising the highway profile by as much as 10 feet in some areas to improve driver sight distance. To accomplish the elevation (and other project elements), the team will place more than 200,000 tons of asphalt and move more than 200,000 cubic yards of earth materials.
Other project elements include:
"We're widening the two existing bridge structures in the southbound direction to accommodate for future HOV lanes," O'Donnell said.
One of those structures is an overhead bridge with the Sonoma-Marin Area Rail Transit (SMART) rail service tracks underneath. The team has partnered with SMART to ensure minimal impact on the passenger rail service.
The last segment of the MSN project has had its challenges. O'Donnell points at the existing subgrade material as a significant issue that the contract has dealt with.
"After removing the asphalt and base material of the old freeway, the existing native material in many areas throughout the project limits was found to be soft clayey and silty material with varying groundwater levels," O'Donnell said. It could not support the weight of the traffic that will regularly use the HOV lane.
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"In many cases, the project had to over-excavate the native material and place subgrade enhancement products as well as additional aggregate base material in order to construct an adequate new pavement section," O'Donnell said.
The team made sure to use salvageable materials. The contractor utilized a large Powerscreen impact crusher to fracture and shape the existing concrete and asphalt material from the old freeway into processed sand and gravel that they can then use to incorporate into the new freeway roadway base section.
"The processing of concrete and asphalt that was called to be removed from the old roadway, and then incorporating it as aggregate base material for the new roadway, significantly reduces the amount of virgin material to be produced for the project and eliminates trucking and environmental impact that would have been needed to dispose of the old highway materials," O'Donnell said.
Another challenge is minimizing lane closures since there are already high traffic volumes. Regular lane closures would make the standard congestion drivers face unbearable. This concern/issue was considered when the design team determined staging.
"Multiple construction stages were planned to reconstruct the freeway without reducing the number of open lanes to the public," O'Donnell said. "This increases the duration of construction but significantly reduces the traffic impact due to construction activities."
Ghilotti Construction Company has worked with Caltrans many times over the years. They are regularly on Caltrans projects in the North Bay area and won this contract based on being the lowest responsive and responsible bidder.
The company is headquartered in Santa Rosa, California, (located in Sonoma County) and a few miles north of the project limits. Ghilotti was founded in 1914 and is a 100 percent employee-owned business.
O'Donnell said that Ghilotti has "brought with them their years of experience in similar freeway widening projects, with a dedicated field and office team, which continues to work in partnership with Caltrans."
The construction budget for the project was nearly $92 million. The funds are being provided by the following:
O'Donnell said the team is within range of its current budget. He attributes this to "continuing to productively work through the winter months. Due to weather and atmospheric river events, some work had to be postponed, but we stayed on target."
Construction on the current section of the project began in the summer of 2022. Caltrans gave the team 750 working days, or three years, for construction activities. The team has one additional year for plant establishment.
During the plant establishment stage, the contractor remains on site to inspect and maintain the project’s newly placed permanent erosion control features. Over the newly constructed embankments along the highway, the contractor applies a mix of native grass and flowers toward the end of the construction activities. This is done to stabilize the disturbed-soil areas that are impacted by the highway widening work.
During the one additional year of plant establishment, the contractor is responsible for watering and weeding as necessary, in order to ensure proper germination and growth.
"We are very close to being on track for the original schedule, with opening the HOV lanes targeted for the end of summer 2025," O'Donnell said. "We've managed this by working on critical work during winter and minimizing construction delays and lost time."
He added that challenges related to the native base material, temperature, and weather restrictions on certain operations (asphalt paving) have caused some relatively minor delays to the work progress.
When the final MSN project is completed, drivers will have access to a new, widened road. The Marin-Sonoma Narrows will not be so narrow. The route will be a controlled-access highway with no traffic signals or intersections, easing travel congestion in the San Francisco suburbs.
