After the Alaskan Way Viaduct has been replaced, West Seattle commuters may see their commute times to downtown double, making it as efficient to take the bus as drive.

Travel times - one measure of how well each of the eight proposed scenarios replace the viaduct - were unveiled at the Alaskan Way Viaduct Stakeholders Committee meeting last Thursday.

Jim Parsons, the manager of the independent team overseeing the project, presented numbers from the traffic models, showing the predicted numbers of trips people will take through downtown in 2015, based on each of the scenarios.

Parsons also spoke about the predicted loss of parking downtown, movement of freight from the Port of Seattle terminals, improvement of routes for bicycles and better connections between neighborhoods.

Results from each traffic simulation was compared to a "no build" scenario, imagining the viaduct demolished in 2012 and replaced only with a four-lane waterfront highway, with no other improvements to streets, transit or the freeway. The 2015 baseline includes predicted population and employment growth, and its increased traffic, for a better comparison than against today's numbers.

The results showed driving times from West Seattle to nearly double, taking 13 minutes to commute downtown today, and 20 to 25 minutes under the proposed scenarios. Driving through the no-build baseline would take 30 minutes.

The numbers are biased however because of the destinations used in the traffic simulation. Today, drivers from the Junction take the viaduct directly to First Avenue and Seneca Street. In the future, all eight scenarios eliminate ramps at Seneca and Columbia streets. The simulation included the extra time drivers spent getting to First and Seneca, through Pioneer Square, from the King Street exits from the new southern half of the viaduct.

"The times would be flip flopped (between the two locations)," Parsons said. "It will be a lot quicker to get to Pioneer Square than it is today."

Transit will also take longer than it does today, 21 versus 25 minutes, even with Metro's new RapidRide route from West Seattle and a dedicated lane on the new south half of the viaduct. The traffic model showed drivers shifting to transit as the time to take the bus became close to the time to drive.

The number of vehicles using any of the alternatives will decrease.

Today, about 30,000 vehicles pass Yesler Way on the Alaskan Way Viaduct during a three-hour afternoon peak period. Scenarios predict 11,000 vehicles (on a surface boulevard) to 23,000 vehicles (through a bored tunnel) will pass the same point on whichever alternative replaces the viaduct.

Total trips through the whole city, though, aren't predicted to change much, as commuters switch to buses, drivers take surface streets, and shoppers avoid going through downtown.

All drive times, and some transit times, worsen. Only transit to the airport improves because Sound Transit's light rail will be open from downtown, and from the University District in 2016.

Regardless of what replaces the viaduct, the models show increased use of transit, as driving takes longer, and more people walk and bike especially by the anticipated increase in downtown residents.

Driving times through the lidded trench will not be a quick as through the other two tunnels or on the two elevated options, because a lowered highway will return to the surface at the north and south ends of the waterfront, to face street intersections - and red lights.

The two elevated scenarios best preserved driving times through the city.

"If you're looking at a particular trip, it may seem (these are the most efficient)," said Ron Paananen, the deputy director of the Urban Corridors Office at Washington State Department of Transportation. "But (this modeling) is intended to have a wider focus. And we have five other guiding principles to satisfy."

Engineers for the Washington State Department of Transportation developed the computer models of traffic, studying a networked grid of streets from 85th Street to the southern city limits, from Elliott Bay to Lake Washington. They focused on traffic using the Alaskan Way corridor and Interstate 5, for people coming to and leaving from, or driving through, downtown.

Thursday's meeting had been postponed twice. Some traffic simulations took days to complete, said Harold Taniguchi, director of the King County Department of Transportation.

Traffic models presume the number of households in the region will increase 30 percent and employment by 15 percent by 2030, based on predictions provided by the Puget Sound Regional Council. The Washington State Department of Transportation calculated daily trips - by car, car pool, bus, streetcar, light rail, bicycle and by foot - would increase 20 percent.

The traffic models aren't meant to be predictive of actual trips made in 2015, but to produce numbers to compare the scenarios against each other.

"None of this is the final answer," Paananen said. "(The numbers) are meant to inform, to help us come up with the best answer."

The models did accurate predict 2008 traffic patterns, validated against actual counts of vehicles done this year.

The eight scenarios are: two options of surface streets replacing the viaduct, one with added transit and the return of the waterfront street trolley; a surface street couplet with Alaskan Way and Western Avenue; a four-lane, side-by-side, elevated roadway; an integrated elevated roadway running through a structure with retail and residential in the base and acres of park on top; a bored tunnel bypassing downtown; a cut-and-cover tunnel; and a lidded trench roadway along the waterfront.

None of the eight scenarios is a full-capacity replacement for the existing Alaskan Way Viaduct. All will connect to the new construction replacing the southern half of the viaduct, from Holgate to new exits at King Street. The scenarios also include a variety of enhancements to the street grid, parking, transit and I-5.

Each scenario will be measured, comparing how well it moves people and freight across the region, versus its cost to build.

"An eight-lane replacement would be the most efficient," said Paananen. "But it would not be cost effective."

The committee will meet weekly to finish its work by the end of the year.

The costs to build each of the eight scenarios will be announced at the Nov. 20 meeting. The economic impact to businesses - another of 27 measures evaluating the replacement for the viaduct - will be discussed at a December meeting.

The most effective enhancements from each will be recombined into three "hybrid" scenarios for a final evaluation. The directors for the city, county and state departments of transportation plan to make a recommendation to the mayor, county executive and governor by the end of the year.

More than $1 billion, of $2.8 billion committed by Gov. Chris Gregoire for a six-lane replacement to the viaduct, has been used for stabilizing footings, moving utilities off the structure, improving fire safety in the Battery Street Tunnel and building a new southern half of the viaduct from Holgate to King streets. More funding may be requested from the state Legislature in January.

Viaduct replacement images on Web

What might a replacement to the Alaskan Way Viaduct look like?

The Washington Department of Transportation has posted images on the Web, computer-modeled illustrations comparing the eight scenarios - three surface alternatives, two elevated roadways, three tunnels - by what they look like from the aquarium, at Pier 54, and from aerial views above Pioneer Square, the waterfront and Aurora Avenue.

The images can be found at http://www.flickr.com/photos/wsdot/collections/72157609012554549/.

Matthew G. Miller is a freelance writer living in the Admiral District and he may be reached via wseditor@robinsonnews.com.