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.”