All Washington’s taxpayers will be on the hook for any cost overruns for the tunnel that is now supposed to replace the Alaskan Way Viaduct on downtown Seattle’s waterfront – if there are any.

“There won’t be any cost overruns,” state Transportation Secretary Paula Hammond promised Monday during an interview with The News Tribune.

Hammond and David Dye, deputy transportation secretary, said they are confident the state’s portion of the joint project – a 1.7-mile tunnel under First Avenue and most of the improvements to Alaskan Way surface street – will cost $1.9 billion or less. That’s in addition to the $900 million the state already has committed to the project, putting the total state obligation at $2.82 billion.

Dye and top transportation officials for King County and the City of Seattle gave members of the Senate Transportation Committee a briefing Monday on the deal struck two weeks ago by Gov. Chris Gregoire, Seattle Mayor Greg Nickels, King County Executive Ron Sims and the Port of Seattle for a $4.24 billion replacement for the earthquake-damaged viaduct.

The governor and local officials are asking the Legislature to approve the agreement and pony up the rest of the money for the state’s share of the project, and give more taxing authority to King County. Seattle also may need some legislation to form a local taxing district, but that is not yet clear.

The state wants to hire a designer-builder for the tunnel by mid-2011, “so we have to leave here with a financing plan in hand,” Dye said.

That means the Legislature would have to come up with an additional $400 million, beyond the $2.4 billion that lawmakers already have committed to the project. The source of that money is not certain, but Sen. Mary Margaret Haugen, D-Camano Island, committee chairwoman, knows where she wants to get it: tolls.

Dye told committee members a cursory study suggests the state could raise between $350 million to $400 million by placing a toll on the tunnel.

Harold Taniguchi, King County’s transportation director, said the county needs state lawmakers to approve a 1 percent motor vehicle excise tax only for his county to raise enough money to pay for its share of the project, and other transit projects. More bus service is part of the project.

“We can’t play without it,” he said. He told the committee it would cost the average vehicle owner about $75 a year, based on the 1.6 million vehicles that were registered in King County in 2007. The total is $120 million a year.

It was Dye who also delineated each jurisdictions areas of responsibility on the overall project, referring to a matrix with columns for the state, city, county and port. That includes any overruns. Seattle is responsible for replacing the central seawall along the waterfront, part of promenade on the surface street, moving utility lines and some transit and street work. King County would share in transit service and street work.

“If it’s in your column, you’re responsible for it (overruns),” he said. “If it’s less, you’re done.”

Dye said the cost of a bored tunnel is now pegged at $1.2 billion on the low end and $2.2 billion on the high side. Hence, the Department of Transportation is comfortable with the $1.9 billion estimate.

Part of the appeal of a bored tunnel is that it would not disrupt traffic on the existing viaduct, which now carries about 110,000 cars and trucks a day. The tunnel would start in the vicinity of the Mariner and Seahawk stadiums and emerge at about Denny Way and Harrison Street, reaching depths of up to 250 feet in places.

Dye devoted a portion of his presentation to why Seattle’s tunnel would not turn in a “Big Dig,” a waterfront tunnel project in Boston that started out as a $2.8 billion project but is now pegged at $22 billion. The Big Dig was 8 miles; this one is 2.8 miles. The Big Dig has three tunnels; this one has one tunnel. The Big Dig has a total of more than 161 lane miles; this one has less than 13 miles (the Seattle tunnel will have four lanes).

Also, the Seattle tunnel will have only one agency in charge, the Department of Transportation, whereas the Big Dig had a consortium of governments running the project, he said.

Joseph Turner: 253-597-8436 blogs.thenewstribune.com/politics