US-101 Marin-Sonoma Narrows Carpool Lanes Update
UPDATE on November, 5 2025: We understand that the change to the hours of operation on the HOV lanes on US-101 has led to some congestion and delays as motorists start to adapt to them. Additional factors, such as the closing of the gap on the HOV lane between Novato and Petaluma and the elimination of the clean air vehicle access also appear to be contributing to these impacts. It is a standard traffic engineering practice to allow some time for traffic patterns to stabilize following operational changes. Caltrans has been working closely with our transportation partners to monitor and evaluate the performance of the freeway in an expedited way. If our analysis shows that changes are needed, we will work with our partners to implement them.
UPDATE on October 1, 2025: Vehicles displaying a Clean Air Vehicle (CAV) decal will no longer be allowed to travel in carpool High Occupancy Vehicle (HOV) lanes with a single occupant or receive reduced toll rates in some areas. To help make this transition easier for the traveling public in Marin and Sonoma counties, Caltrans and the California Highway Patrol (CHP) have agreed to extend a grace period for enforcement until Dec. 1, 2025. For more information about enforcement issues, please contact CHP.
UPDATE on September 8, 2025: Caltrans finished installing signs with new HOV hours on Saturday, Sept. 6. The new hours took effect on Monday, Sept. 8. This is a change from what was reported previously as Caltrans is striving to make this transition easier on the traveling public.
A third lane for the Marin Sonoma Narrows B-7 project is scheduled to open in the Novato area of Marin County in late September. Electronic message boards will continue to be activated to help motorists along this corridor.
The new HOV hours have changed, and Caltrans will continue to monitor the corridor through this period. This is a transition period for the traveling public getting used to the new HOV lane hours, and Caltrans did expect some initial delays.
HOV lane hours to be uniform in Marin, Sonoma counties
Signs along the existing HOV lanes in Marin and Sonoma counties have been modified or replaced with signs displaying the new hours.
Upon completion of the Marin Sonoma Narrows project in 2025, there will be 52 miles of continuous carpool (high occupancy vehicle, HOV) lanes on US-101 between the Richardson Bay Bridge in Marin County and Windsor in Sonoma County. There will only be one HOV in each direction of US-101. The new HOV lanes, when complete, will be the second longest in California.
Now that the gap in the HOV lanes through the Marin Sonoma Narrows will be closed, the hours of operation have been revised to ensure consistency.
HOV lane hours and directions so that they are the same in both counties will fulfill the commitments made in the Environmental Impact Report (EIR) for the project.
The new hours will be 5 to 10 a.m. in the mornings and 3 to 7 p.m. in the evenings, Monday through Friday. Signs along the existing HOV lanes in Marin and Sonoma counties will be modified or replaced with signs displaying the new hours. HOV lanes reduce traffic by taking off of the road as people switch to ridesharing or using transit. Some of the most heavily used HOV lanes can carry one-third of the people utilizing a freeway during a peak period. HOV operating hours are typically set to cover the periods of peak congestion and short time blocks leading up to and after those peak periods.
The HOV hours for US-101 were determined through an analysis of traffic data that was collected in the summer of 2024.
The Freeway Management Executive Lane Committee, comprised of Caltrans, the Metropolitan Transportation Commission (MTC) and California Highway Patrol (CHP) met to finalize the final hours. This was after coordination with Transportation Authority of Marin (TAM), Sonoma County Transportation and Climate Authorities (SCTCA) and MTC. At this time there are no plans to convert the HOV lanes into express lanes.
As part of the changes to the HOV lanes, Caltrans activated on-ramp meters at four of the busiest northbound ramp locations to improve traffic conditions in June, 2025. These meters are located at Gate 6 Road/Bridgeway, East Blithedale Avenue, Tiburon Boulevard and Tamalpais Drive. Caltrans and partners are working toward activation of corridor adaptive ramp metering at other ramps in Marin and Sonoma counties to help with traffic flow in late 2025/early 2026.
Ramp meters are effective to reduce total travel delay, provide consistent travel times, provide HOV incentives when warranted, break up platoons of vehicles, making merging safer and reduce emissions.
Additional ramp meters along the US-101 corridor will be activated in phases and more information will be provided as details become available.
Traffic Monitoring
Caltrans and MTC are monitoring traffic conditions on the freeway using traffic cameras, vehicle detection stations, and other sources. Staff are looking at various data - including travel times, speeds and congestion duration, transit travel times, transit ridership, corridor traffic volumes, HOV lane utilization, incidents, and freeway service patrol response.
MTC, Caltrans, TAM, and SCTCA are working together to collect and analyze traffic and transit performance on the freeway.
While the traffic analysis is being expedited in response to concerns, it may take time to allow traffic to normalize first before making any adjustments to avoid over-correction. Allowing enough time to observe recurring patterns will ensure that the adjustments are based on the most accurate data. Traffic data will be gathered through February 2026 to have enough information to show trends and help with decision making and consideration of whether the HOV hours will need to be adjusted.
State and Federal Laws Regarding HOV Lanes and Occupancy Requirements
State and federal laws allow the establishment of HOV lanes on public roads. State legislation governing the development and operation of HOV lanes on State highways has existed since 1969, while federal laws with the same objectives were passed in 1970.
The State laws are Section 21655.5 of the California Vehicle Code and Section 149 of the California Streets and Highways Code which grant Caltrans the authority to establish HOV lanes on State highways, and to designate the vehicle occupancy requirements, and the hours of operation.
The primary Federal law governing HOV lanes is Section 166 of Title 23 of the United States Code. This law governs what vehicles are allowed to use HOV lanes.
Other sections of Federal law pertaining to HOV lanes include Section 7408 of Title 43 of the United States Code, which lists HOV lanes as an acceptable transportation control measure for states failing to meet federal air pollution standards, and Section 149 of Title 23 of the United States Code which authorizes the use of federal transportation funds to support the development of HOV lanes.
The legislative intent for HOV lanes in California is spelled out in subparagraph (e) of CVC 21655.5: "It is the intent of the Legislature, in amending this section, to stimulate and encourage the development of ways and means of relieving traffic congestion on California highways and, at the same time, to encourage individual citizens to pool their vehicular resources and thereby conserve fuel and lessen the emission of air pollutants. HOV lanes are intended as an incentive for people to not drive alone, and in so doing, they also help California meet federal air quality standards by helping to reduce the number of single occupant vehicles on roadways."
Project Newsletter
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Marin Sonoma Narrow project contacts:
Matt O'Donnell
Marin County Public Information Officer
Email: matt.odonnell@dot.ca.gov
Jeff Weiss
Sonoma County Public Information Officer
Email: jeffrey.weiss@dot.ca.gov