Tolls on the Highway 520 bridge between Seattle and Bellevue would have to be twice as high as the $3 toll on the Tacoma Narrows Bridge if tolls were used to pay the bulk of the cost of building a six-lane replacement bridge.

And the tolls probably would have to remain in place nearly twice as long as those on the new Narrows bridge.

That was the news delivered Monday in Olympia to members of the House Transportation Committee, which got a briefing on the state’s two most vexing mega-projects – the Alaskan Way Viaduct and the Lake Washington floating bridge.

The “best-case scenario” for putting tolls on the floating bridge is a round-trip fare that would range between $5.66 and $8.13 in today’s dollars. That compares to a $3 round-trip toll on the Narrows Bridge, which is expected to be imposed in July.

But since the Highway 520 tolls most likely would not be put in place until the new, wider bridge opened to traffic in 2015, the average toll in that year would be between $6.90 and $9.90.

Tolls would be at the low end of that range only if the state also levied the same toll on Interstate 90, the other lake crossing, and only if the state borrowed money for 40 years instead of the 30-year period for most highway projects. That would lower the loan payments, much like a 30-year home mortgage has a smaller payment than a 15-year mortgage, so the tolls could be lower.

Although the Alaskan Way Viaduct has been getting most of the attention from lawmakers and Seattle officials in recent weeks, it has an advantage over the Highway 520 bridge: Most of the money for the viaduct already has been appropriated or identified.

Not so with the floating bridge.

The new cost estimate for the Highway 520 bridge is $4.38 billion. The Legislature so far has committed only $552 million. Last week, the Regional Transportation Investment District in Pierce, King and Snohomish counties pledged $1.1 billion toward the bridge – if voters approve tax increases to raise $8.5 billion for road projects.

But that still leaves a shortfall of more than $2.7 billion – plus another $200 million if the state decided to perform work on the I-90 bridge so it also could be tolled.

The $200 million would prepare the I-90 bridge for the addition of a light-rail line or buses between Seattle and King County’s Eastside.

Jeff Doyle, director of the Public-Private Partnership program for the state Department of Transportation, said Gov. Chris Gregoire directed DOT officials to see whether tolls could fill the shortfall. An expert review panel that examined the project also urged a closer look at tolls.

None of the scenarios for tolling only the 520 bridge passed muster of DOT officials, investment bankers or private investors, Doyle said.

A 50-year project would have a $10 toll. A 75-year project would have an $8.50 toll. And the conventional 30-year project would have a $16 toll when the bridge opened in 2015.

DOT officials say that last scenario was “not realistic,” Doyle said.

Tolling both Lake Washington bridges for 40 years was the “best case for toll financing,” Doyle said.

Rep. Alex Wood, a Spokane Democrat and a member of the transportation committee, said members first saw the numbers only a week ago, so they haven’t had time to digest them.

Rep. Dennis Flannigan, D-Tacoma, committee vice chairman, said the tolling scenario is in the mix of options that lawmakers are looking at.

“I don’t think we can go home without responding to the overwhelming cost of the project,” Flannigan said.

Tolls on the second Narrows Bridge may be as low as $1.75 this summer for motorists who install transponders that allow electronic collection of tolls, and remain that low for the first year. But the tentative schedule for increases calls for an average toll of $4 in 2010, $5 in 2013 and $6 in 2016. That $6 toll would remain in place until 2030, when the bridge project is expected to be paid off.

That would be 23 years. By comparison, if the state used 40-year bonds to pay for the Highway 520 bridge project, they would begin in 2015 and would not be paid off until 2055.

Borrowing money for as long as 40 years would require a change in the Washington Constitution, which would mean voter approval. But lawmakers in both the House and Senate have proposed the longer loan periods to cover higher-than-expected costs for many projects that were supposed to be built with a 14.5-cent increase in the gas tax approved in phases since 2003.

Joseph Turner: 253-597-8436

joe.turner@thenewstribune.com