For the past 13 years, Highway 101, the main artery through Sonoma County, has undergone the biggest transformation since the route was built in the 1950s. The highway is being widened from Windsor to Highway 37 in Novato to add a third lane in each direction and help ease traffic.

Construction has inched north from Marin County and south from Windsor and now just the section from Petaluma to northern Novato remains. Some work is currently underway in the section known as the Sonoma-Marin Narrows, but that work will only add frontage roads and widen bridges in preparation for a third lane. The actual widening won’t happen until all of the work is funded, and two sections — the stretch through Petaluma at $90 million and from South Petaluma Boulevard to the county line at $35 million — remain unfunded.

The Sonoma County Transportation Authority moved the Petaluma project forward on Monday by approving $4.5 million to purchase properties needed to widen the freeway. Officials say that using local sales tax money to get the projects ready for construction has been a successful strategy to attract state and federal money to build the highway lanes.