Driving State Route 37 Across the Marsh

Highway 37 is the North Bay's shortcut between Marin, Sonoma, Napa and Solano, and it runs across some of the flattest, wettest ground in the region. Most days it is simply slow. A few days a year, the marsh decides the road is part of the bay. Commuters and weekend visitors alike should understand both.

Why this road behaves differently

Caltrans describes SR 37 as the North Bay's most heavily used east-west highway, a 21-mile corridor connecting I-80 and US 101 through Sonoma, Marin and Solano counties. Much of it crosses one of the Bay Area's largest remaining tidal marshes, the San Pablo Baylands. The road is affected by uneven subsidence, meaning the ground beneath it settles at different rates, and by intermittent storm-related flooding in several areas.

The long-term picture is stark. Caltrans says nearly the entire stretch between Novato and Vallejo is predicted to become permanently submerged as sea levels rise if modifications are not made, and that early conclusions identified portions that will be completely inundated by 2050, with more frequent flooding on the way there. In 2015 the four North Bay counties' transportation authorities signed an agreement to pursue rebuilding the corridor to withstand rising seas and storm surges.

The bottleneck

Between SR 121 and Mare Island the highway narrows from four lanes to two, and Caltrans identifies that section for large delays. It is busy on weekday commutes and again on weekends, when the route fills with people heading to and from the Napa and Sonoma wine country. Long stop-and-go queues on a two-lane road with marsh on both sides produce the usual results: rear-end collisions, frustrated merges and risky passing. Leave space, expect sudden stops at the merge, and do not treat the shoulder as a lane.

When water is on the road

Flooding here tends to arrive with heavy rain and high water in the bay. If you see water across the lanes, do not drive into it. On this road the water can hide the edge of the pavement, and there is often nowhere to go if the car stalls. When the highway is closed, the alternatives, inland through Petaluma and Sonoma or around by way of the Richmond-San Rafael Bridge, add real time. Check conditions on Caltrans QuickMap before a storm-day trip and decide your fallback before you are sitting in the queue.

Flood damage and your policy

The California Department of Insurance has said plainly that comprehensive vehicle insurance can cover flood damage and other winter storm damage, and urges drivers to consider it. A liability-only policy pays nothing toward your own car, flooded or otherwise.

How a claim is handled can depend on how the water got into the car. A car swamped while parked and a car driven into standing water may be treated differently, and that varies by insurance company, so read your own policy. Mechanics commonly advise against trying to restart an engine that has taken in water, because doing so can turn a repair into a replacement. Photograph the waterline inside and out before anything is moved, and call the claim in promptly.

The long view for regular drivers

Rebuilding a corridor like this takes years, and projects of that scale tend to bring their own lane closures and detours along the way. If you commute across 37 from Solano to Marin, it is worth having a standing plan B, a roadside program that will reach you on a causeway, and comprehensive coverage if your car is worth protecting. You can check where your policy stands in a couple of minutes.