Drivers who use State Route 37 between Vallejo and Sonoma should plan around a multi-day full closure later this month. Caltrans District 4 says both eastbound and westbound lanes will shut from Mare Island to Sears Point from 10 p.m. Friday, Oct. 23, until 5 a.m. Monday, Oct. 26 — about 55 hours — so crews can prepare the corridor for flooding tied to a forecast El Niño season. The agency posted the schedule Oct. 13; the Daily Republic reported the same window Wednesday.

All work is weather dependent. Caltrans has also scheduled westbound-only overnight closures before the big weekend: 7 p.m. to 4 a.m. on Wednesday, Oct. 21, and again Thursday, Oct. 22.

What crews are doing

During the 55-hour shutdown, crews will demolish part of the median and install temporary concrete barrier (K-rail) so traffic can later be shifted onto the westbound side. Once that shift is set, work focuses on the eastbound lanes: Caltrans says portions of the roadway will be raised by about three feet to improve flood resilience along a stretch that has repeatedly flooded in past storms, including in January 2017.

A second, shorter full closure is already on the calendar for barrier work: from 10 p.m. Friday, Nov. 13, to 5 a.m. Sunday, Nov. 15, crews plan to remove temporary barrier on the westbound side and shift traffic and barrier onto the eastbound side. That window can also move with weather.

Detours Caltrans published

Eastbound: Exit left onto northbound Highway 121 in Sonoma, turn right onto Highway 12 / Carneros Highway, continue, then turn right onto Highway 12 / Highway 29 and use the connector loop back to eastbound 37.

Westbound: Exit right onto northbound Highway 29 in Napa, follow Highway 29 to westbound Highway 12, then south on Highway 121 back toward westbound 37.

Expect longer travel times on those two-lane roads, especially if rain or weekend wine-country traffic piles on. Caltrans has posted changeable message signs and points drivers to QuickMap and 511.org for live conditions.

Why this corridor matters

Highway 37 is the short cut across the northern edge of San Pablo Bay between Solano County and Sonoma County. When it closes, Vallejo–Marin and Vallejo–Sonoma trips spill onto Highways 29, 12 and 121 through Napa and the Carneros. Freight, weekend visitors and daily commuters all feel it. For other North Bay travel and weather coverage, see our stories on the Santa Rosa to SFO Landline bus starting Nov. 17, the I-80 crashes at Fairfield's Air Base Parkway, the PG&E Public Safety Power Shutoff watch for Sonoma, Napa and Solano, and San Francisco's separate Highway 101 Hospital Curve closure Oct. 16–19.