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Originally published Tuesday, January 13, 2009 at 12:00 AM

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Who'll pay? Another "Big Dig"? When will it open?

Here are some answers to questions you might have about the bored-tunnel concept to replace the Alaskan Way Viaduct.

Seattle Times transportation reporter

Here are some answers to questions you might have about the bored-tunnel concept to replace the Alaskan Way Viaduct:

Q: If the state has $2.8 billion, where would the money come from to pay the total project cost of $4.25 billion, including transit and street improvements?

A: From you. Beyond that, there were few details Monday.

King County, Seattle or other local interests would have to pay for street work and other related features beyond the tunnel itself, according to Ron Judd, senior adviser to Gov. Chris Gregoire. The state figures the $2.8 billion would cover the cost of the tunnel itself and the mostly elevated highway segment in Sodo.

And actually, the state might have only $2.4 billion for the viaduct replacement because lawmakers (for now) have shifted $400 million to building a new Highway 520 floating bridge.

Conceivably, several potential taxes and fees could be considered: a neighborhood property tax in downtown Seattle to pay for parks, walkways or streetcars; a county car-tab fee for Metro Transit; regional tolls for improvements on Interstate 5. Or, those ancillary projects might be cut.

Q: Everyone knows about Boston's ultra-expensive "Big Dig." Will there be cost overruns here?

A: Could happen. Tunneling is generally the most expensive form of highway construction.

Less than 1 percent of engineering is done here, and some soil in the tunnel route hasn't even been sampled. Industry practice is to distrust estimates until a megaproject is at least 30 percent designed.

Last year, the state had to boost its Highway 520 bridge estimate by $700 million while reducing its option for an elevated viaduct by two lanes — due to rising materials prices and design changes.

When Sound Transit started building tunnels at Beacon Hill, the sole bid came in $41 million over estimates, and costs have risen $20 million since, for a total $302 million.

Inflation should ease in the current recession. And a bored tunnel, perhaps 200 feet deep, might be more predictable than a shallow, cut-and-cover tunnel or elevated highway along Elliott Bay, where builders must fend off seawater and build detours.

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Another caveat is "optimism bias," as information arrives from contracting firms with a financial interest in tunneling, or from agencies whose leaders are eager to create political support.

Q: Will a tunnel carry as much traffic as the viaduct?

A: Not quite.

A bored tunnel would lack ramps at Western Avenue to serve Belltown, Interbay and Ballard, areas reached by the current viaduct. Traffic from those places, including trucks, would need a surface road near the waterfront, or would crisscross neighborhoods to enter the highway at Aurora Avenue North.

The tunnel would have two lanes in each direction, compared to three on the viaduct. On the other hand, the new tunnel could move at least as many cars through downtown as today's four-lane Battery Street Tunnel, which carries 63,000 vehicles per weekday.

The existing viaduct carries 109,000 vehicles on weekdays at its busiest point, mid-downtown. In the new project, pressure on traffic downtown would be relieved somewhat by a new Sodo interchange.

Q: How would a tunnel connect to Aurora Avenue?

A: It would emerge just north of the Battery Street Tunnel, which might become an ordinary city arterial, predicted Ron Paananen, project manager for the state Department of Transportation. Aurora would be widened somewhat to create South Lake Union offramps, near the tunnel mouth.

One question is how this affects east-west Mercer Street at this junction. Could the city still convert Mercer to a tree-lined city boulevard — or should it be a roaring connector between Highway 99 (Aurora) and I-5? "I think the Mercer issue is still up for debate," said King County Councilmember Larry Phillips, D-Magnolia.

Q: How long before a tunnel opens?

A: The state might aim for August 2017, according to a chart issued last month.

One advantage to the bored tunnel is that the old viaduct can carry traffic during tunnel construction, backers say. But Gregoire has said many times that the old viaduct is coming down in 2012 because it's unsafe. So is she going to stretch her deadline? Reinforce the old columns? Argue that a tunnel can be done faster? Her office had no comment Monday. Perhaps, all of the above.

Mike Lindblom: 206-515-5631 or mlindblom@seattletimes.com

Copyright © 2009 The Seattle Times Company

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