Replacing and widening the Highway 101 bridges over the Petaluma River is the next major step in a billion-dollar freeway project that has proceeded in spurts over the past decade — and is now within a few years of completion.

The state is expected Wednesday to approve $82 million for construction that could begin this fall to replace the parallel spans over the Petaluma River that are one of the bottlenecks on the highway from Windsor to Marin County.

Also this fall, two of the most critical segments long under construction should open: Replacement of the Highway 101 overpass of Wilfred Avenue in Rohnert Park at a cost of $65.4 million and the widening of the freeway from Rohnert Park Expressway south to Petaluma Boulevard North at $148.2 million.

And work also should begin on a new $34.5 million interchange of Highway 101 at Airport Boulevard.

All of this construction over the next two years would leave completion of $950 million widening short by about $177 million.

"The end is in sight," said James Cameron, Sonoma County Transportation Authority's deputy director of projects and programming. "We would like to see the whole thing done by 2021, whether it will take a sales tax extension, a new sales tax or state of federal funding."

The funding gap, however, is a big one.

All of the money from the Measure M quarter-cent sales tax for Highway 101 has been spent or committed by Sonoma County to repay construction bonds, leaving nothing left to leverage additional state and federal funds.

The Transportation Authority so far has committed $140 million to the Highway 101 widening and has received $615 million in state and federal funds.

The hope is that Sonoma County's investment and work will be recognized by state and federal officials when additional funding becomes available, said Suzanne Smith, executive director of the Transportation Authority.

"We have put a lot of local money into the corridor, and we have to work with our state and federal partners and say lets finish this off, lets close the gap," Smith said. "We have 20 percent to go, basically."

Widening Highway 101 to six lanes has been under way since 2002, when a five-mile stretch opened between Highway 12 in Santa Rosa and Wilfred Avenue in Rohnert Park, at a cost of $29 million.

The highway since was widened through downtown Santa Rosa from Highway 12 to Steele Lane at $149 million and from Steele Lane to central Windsor at $158.5 million.

Sonoma County financed its portion of the widening, including environmental studies and design, with the tax revenues and by selling $63.5 million in bonds, which are being repaid by the sales tax.

Measure M, passed in 2004, is a quarter-cent sales tax that designates 40 percent for Highway 101 widening. The rest is for local roads, transit, SMART and bike and pedestrian projects.

In the first seven years of the 20-year tax, $121 million has been raised.

The replacement of the north and southbound Highway 101 bridges over the Petaluma River also could be under construction this October if the state Transportation Commission approves $82 million in funding as expected today. It is a two-year project.

In June, the commission also may approve $32.4 million to replace the Highway 101 overpass over Highway 116, just north of the Petaluma River bridges.