On the day the new Graton Resort & Casino opened last week, northbound traffic on Highway 101 backed up from Rohnert Park all the way to Novato. It took drivers an hour to travel just 22 miles.

The casino is expected to add up to 10,000 vehicles per day to the already overburdened artery, highlighting the need to complete a long-planned Highway 101 overhaul, widening bridges, reconfiguring interchanges, and other improvements.

"A traffic generator like the casino makes this project even more important," said Dianne Steinhauser, executive director of the Transportation Authority of Marin. "It emphasizes the need to do it."

Much of that work will be completed by 2017.

But even after all the improvements, the worst bottleneck in the area will remain: the freeway will still narrow from four lanes in Marin County down to two lanes from north of Novato to Petaluma. A lack of funding will leave that 10-mile gap in carpool lanes for at least seven more years, transportation officials say, and frustrate commuters who battle heavy daily traffic between Sonoma and Marin counties.

Thirteen years after embarking on the project to improve Highway 101 from Highway 37 in Novato to Windsor River Road — a nearly 40-mile stretch — construction crews are working on the last section between Petaluma and northern Novato known as "The Narrows."

The projects underway include improving all four interchanges in Petaluma, widening the overpass at Redwood Landfill, adding frontage roads through The Narrows, closing off unsafe highway access points, and widening the bridges over the Petaluma River and Highway 116. All of the work will be completed by 2016.

A future project to realign the freeway and build a new bridge over San Antonio Creek at the county line is fully funded and will break ground in 2015 with a two-year timeline.

When all the work is complete, the freeway will be safer and more modern, but no wider than it is today. Funding for the last piece — a carpool lane in each direction from northern Novato to Old Redwood Highway in Petaluma — is still $250 million short, and officials don't know when it will come, or even who will pay.

"Where's the money going to come from? We don't know the answer to that yet," said Suzanne Smith, executive director of the Sonoma County Transportation Authority. "If someone wrote me a check tomorrow, we could have the whole thing done in four years. Optimistically, we could see funding in two to four years."

The six miles of unfunded Sonoma County carpool lanes are estimated to cost $125 million. Marin County's four-mile gap is $110 million short.

The Federated Indians of the Graton Rancheria, which owns the casino, will pay $2 million over the next 10 years for transportation infrastructure to offset the increased traffic from the gambling palace.

So far, the Sonoma County side of the project has been funded by Measure M, the 20-year sales tax measure voters approved in 2004, and the state and federal money that it has leveraged. But the Measure M fund is largely depleted. Officials borrowed most of the money up front, and the rest of the expected revenue will go toward servicing the debt, Smith said.

Regional transportation planners must now get creative to make up the funding gap.

Extending Measure M at this point is not an attractive option, Smith said, because the cost of borrowing that far ahead is expensive.

Marin County's Measure A sales tax is not supporting the Narrows project, Steinhauser said. Those funds were spent on Highway 101 widening around San Rafael.

Marin is widening its side of the freeway incrementally, including in two spots that should be ready early next year — an additional southbound lane from Novato Creek to Franklin Avenue and a northbound lane from Atherton Avenue to just south of Redwood Landfill.

Agencies are looking for state and federal funding, but Smith doesn't expect money from Sacramento or Washington in the near future.

"In terms of state and federal funds, we don't expect to see a lot of that on the horizon," she said.

A Congressional ban on earmarks makes federal funding for specific projects hard to come by, Steinhauser said.

In the meantime, officials are preparing the highway for the final 12-foot wide strip of asphalt — whenever funds become available — and preparing drivers to expect bottlenecks until at least 2020.

"We made a choice to do the safety improvements first," Steinhauser said. "While we are able to make the roadway safer, we are not able to widen the road all the way at this time. That's going to be frustrating for drivers."

You can reach Staff Writer Matt Brown at 521-5206 or matt.brown@pressdemocrat.com.