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.