The first phase of a massive plan to elevate Highway 37 to prevent regular inundation from sea-level rise is set to begin with an estimated $1.6 billion project in Marin. Caltrans officials held a presentation recently on the agency’s plan to rebuild a 2.5-mile section of the 21-mile North Bay commuter route as an elevated causeway from the Highway 101 interchange in Novato to the Atherton Avenue exit. The project would be the first in the agency’s plan to elevate the entire highway onto a causeway before the road connecting Marin and Solano counties becomes regularly inundated by rising sea levels, which Caltrans projects will begin in 2040.