The Sonoma County Transportation Authority on Monday approved $4.5 million to buy land to widen Highway 101 to six lanes through Petaluma, but it will be at least three years, and perhaps longer, before there is a remedy to the bottleneck that has plagued commuters for years.

One of the two remaining four-lane sections of Highway 101 in Sonoma County from Windsor south to the Marin County line now is nearly ready for construction, though transportation officials have not secured the $90 million needed to widen the highway from just north of Corona Road to the Petaluma River.

The project, which has money for design and environmental work, on Monday got the last piece of funding needed to attract state and federal money for construction, said SCTA executive director Suzanne Smith.

Transportation officials expect to complete the design and environmental work next month. The agency is reviewing an option to phase construction of the project if less than the full $90 million price tag is identified, officials said.

The SCTA money for right-of-way work comes from Measure M, a Sonoma County sales tax. The agency and Caltrans now will start the process of buying land from willing sellers.

Smith said the agency has used Measure M money to leverage hundreds of millions of dollars for highway construction projects.

"Our goal has always been to get projects ready to go," she said. "Throughout the whole 101 corridor, we have worked to position the project to leverage state and federal funds to get the project built."

However, there are no sources of construction funds for at least the next two years, Smith said.

The Highway 101 widening project, begun 13 years ago to add carpool lanes from Windsor to Highway 37 in Novato, has shifted into the final phase with work in the Sonoma-Marin Narrows from Petaluma to northern Novato.

Work is ongoing to widen the Petaluma River and Highway 116 bridges and add frontage roads along Highway 101 through the Narrows. The work is scheduled for completion next year. A project to widen the bridge over San Antonio Creek at the county line is scheduled to start next year and finish in 2017.

All of the work will leave a safer, more modern highway, officials say, but the third lane will not be added until the final two sections around Petaluma are constructed, leaving a bottleneck as motorists merge to two lanes in each direction.

In addition to the three-mile section of unfunded highway through most of Petaluma, a $35 million section from south Petaluma Boulevard to the county line also lacks funds. Like the segment through Petaluma, it has design and environmental funding and is ready for construction once funds become available, Smith said.

The design for the Petaluma project includes a highway overpass for a future crosstown connector at Rainier Avenue, a city project that also lacks funding. Petaluma has long wanted another route across the freeway — the city currently has five overpasses.

The city lost $7.5 million in redevelopment money set aside for the project when a court ruled the city was not entitled to the funds once redevelopment agencies were dissolved in 2011.

You can reach Staff Writer Matt Brown at 521-5206 or