Q&A | Can a four-lane tunnel do the work of six lanes?

At first, the pitch sounds like snake oil: a four-lane waterfront tunnel that can carry as many cars as a tunnel with six lanes and save $1.2 billion.

But just a few days ago, a panel of experts said Seattle Mayor Greg Nickels' last-ditch proposal for a narrower tunnel deserved a closer look by the state.

Gov. Christine Gregoire, however, said enough was enough. She ordered an end to the discussion. Either the state will build a cheaper elevated replacement for the Alaskan Way Viaduct, or more than $2 billion in state aid would be shifted to the equally needy Highway 520 floating bridge.

Nickels and other pro-tunnel city leaders aren't ready to take no for an answer.

They hope a city advisory election would give the tunnel another chance.

Even assuming Olympia cares, the city first has to convince voters that the plan Nickels is now promoting is credible.

Q. How can four lanes carry as many cars as six?

A. During rush hours, the safety shoulders would become exit-only lanes, effectively widening the roadway from four lanes to six. In the morning commute northbound, the right shoulder would become an exit-only lane to Western Avenue. The speed limit would be reduced at peak times.

In off-peak times, the shoulders would serve as break-down lanes, and cars would exit the highway from the usual right lane, leaving two through-lanes in each direction. "The best engineering judgment tells you it would work, but you have to go back and do the analysis," said panel member Don Forbes, a former Oregon state transportation director. The panel was appointed by Gregoire and legislative leaders.

Q. Would traffic become clogged if a car stalls at rush hour?

A. Quite likely.

The city would need to station tow trucks nearby, to clear fender-benders and breakdowns, said Deputy Mayor Tim Ceis. Stalls are rare enough that the occasional tie-up seems a reasonable tradeoff for a possible billion-dollar savings, Forbes said.

At off-peak times, when traffic is moving fastest, there would be a full-sized shoulder, where stalled cars could pull over.

Q. Would a narrower tunnel save money?

A. The city, after seeking data from the state Department of Transportation and engineering consultant Parsons Brinckerhoff and other members of the viaduct design team, produced a cost figure of $3.4 billion. The DOT would not discuss the figure this week.

A four-lane tunnel, with lanes built side by side, would require a shallower trench than the stacked six-lane tunnel. A smaller tunnel would use less concrete. Construction could be finished by July 2013, or 1 ½ years sooner than the big tunnel, the city says.

Q. If this tunnel is so great, why didn't the city propose it sooner?

A. City staffers say they looked at a leaner structure after a price shock Sept. 20, when DOT estimates for the six-lane tunnel increased $1 billion.

Until that point, the city thought a six-lane tunnel was affordable. "Until September 20, we were dealing with a $3.6 billion project," said mayoral spokeswoman Marianne Bichsel.

On Oct. 30, Nickels told KUOW radio a four-lane tunnel might save money.

In mid-December, Gregoire issued a finding that the mayor's funding plan on the original $4.6 billion, six-lane tunnel fell short. She called for Seattle voters to choose between an elevated or tunneled highway — and bear the extra cost of a tunnel.

The city says it recently devised a cheaper four-lane version that connects at Western Avenue — solving an earlier problem that doomed an earlier four-lane alternative.

Q. Did Gregoire act in haste ?

A. The city says it briefed DOT on the four-lane tunnel Jan. 5, kicking off a week of study that included a day of reports to the panel.

But DOT told the panel six days later to halt its review.

"I've been working in infrastructure over 30 years, and I've never seen data on a good idea suppressed in this way," complained a city consultant, Doug Hurley.

In a letter to Ceis and Gregoire this week, the panel says the latest four-lane concept "shows promise."

The panel suggests an independent cost review, before any citywide vote on the future of the highway.

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