Caltrans has proposed a $500 million project to widen a wine country highway that the agency said could be underwater in 25 years.

Members of the California Transportation Commission will decide at a public meeting beginning Thursday whether to award Caltrans and local agencies a $73 million grant that would cover some of the cost to widen Highway 37 — a roadway linking Vallejo to Sears Point across the Napa Sonoma Marsh, much of which is only one lane in each direction.

In the long term, Caltrans has a plan to replace the current road with an elevated causeway that would move vehicles above the wetlands below. That project would cost more than $10 billion and is not funded.

To deal with Highway 37’s bottleneck in the meantime, the agency has proposed a $500 million “interim project” to widen the existing roadway.

The state agency estimated that construction on the first half — a $250 million eastbound lane — would finish in 2029. The plan, Caltrans said, “does not address sea level rise.”

The interim project would ultimately add one tolled lane in each direction as Highway 37 arcs across the northern shore of the San Pablo Bay and plays host to some of the worst traffic jams in the state. The low-lying stretch of highway is vulnerable to sea level rise. Caltrans and the California Ocean Protection Council have said that without intervention, “portions” of the highway “will be completely inundated by 2050.” By that point, two feet of sea level rise is expected.

In an earlier document, Caltrans had a less optimistic projection:

“With the projected sea level rise, most of the existing (State Route) 37 will likely become permanently inundated by the mid-century and even as early as by 2040.”

The interim project would only raise the height of the lanes at a few of the most flood-prone areas: The Tolay Creek Bridge in Sonoma County would be replaced by a structure that’s 5 feet higher, and a 4,400-foot stretch would be raised 8 inches.

Bart Ney, a spokesperson for Caltrans, said that the new lanes would “reduce near-term flooding risks at roadway shoulders near (the Highway 37/Highway 121 split). This is critical to allow the current highway to be functional for as long as possible, while the long-term project is being developed.”

He added that the project “will not only improve near-term mobility for some 40,000 travelers each day but eventually provide essential access and staging areas for the construction crews that will build the elevated causeway through this segment.”

Caltrans has faced criticism from environmental groups for proposing to carve out an exception to state law to allow construction to disturb and possibly harm four vulnerable species that make their home in the wetlands: the salt-marsh harvest mouse, the California Ridgway’s rail — both endangered — along with the California black rail and the white-tailed kite. State law would dramatically limit the number of months per year open to construction, but a Democrat from Suisun City, Assemblymember Lori Wilson, introduced a bill that would effectively exempt this particular project.

Jeanie Ward-Waller, a former Caltrans executive who is now a director at Fearless Advocacy, said the mice and the birds are a mere distraction.

“The bigger concern,” she said, “is just dumping half a billion dollars into the ocean.”

Caltrans and its partner agencies submitted proposals to three grant programs administered by the California Transportation Commission, all of which are funded by a gas tax created in 2017 by Senate Bill 1. Commission staff recommended approving one of the three proposals: a request for $73 million from the Trade Corridor Enhancement Program.

If it won the grant, construction would be scheduled to start in December 2026.

Commuter traffic in particular is brutal on Highway 37. Many workers live in more affordable Solano County but commute to jobs in Sonoma and Marin counties; Caltrans has found that during peak travel times, a drive that would have been 20 minutes without congestion can take an hour and 40 minutes.

Ney said that Caltrans and its local partner agencies found that drivers’ primary concern on the road is congestion.

But there is not enough affordable housing in Sonoma and Marin for workers to simply move closer to their jobs. The Bay Area Association of Governments determined that between 2022 and 2031, the two counties should each add more than 14,400 new housing units. That in itself could cut down on traffic if workers moved in: Highway 37 sees around 30,000 daily motorists, and much of the traffic is fueled by the westbound morning commute and eastbound evening commute.

Commuters are also effectively forced to use cars. No standard bus service traverses Highway 37 because the backups make scheduling too unpredictable. An extra lane, Caltrans said, would allow buses to start ferrying people into Novato, Petaluma and other North Bay cities.

Any bus lines that materialized would be relatively short-lived due to the short projected lifespan of the highway itself. When Caltrans released its Planning and Environmental Linkages study in December 2022, staff wrote that 2 feet of sea level rise was anticipated by 2050, which would permanently inundate multiple areas of the highway. If the area were hit with a 100-year storm — a type of severe storm, the agency noted, that is becoming more frequent with climate change — on top of 24 inches of sea level rise, most of the highway would be underwater.

An environmental report on the interim project estimated that the $10 billion “ultimate” project — the elevated causeway — could be completed by 2050. But the timeline for the unfunded project that would withstand climate change is uncertain.