About 3,200 people said goodbye to the southern part of the Alaskan Way Viaduct Saturday with a stroll along the top deck, even as heavy machines were breaking apart a few road sections a short distance away.

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About 3,200 people said goodbye to the southern part of the Alaskan Way Viaduct on Saturday with a stroll along the top deck, even as heavy machines broke apart sections of the road a short distance away.

After closing off traffic Friday night for nine days, the state opened the viaduct for a public walkabout while crews began demolition on the Sodo portion of the double-decker Seattle highway. It’s the latest and most visible chapter in the $3.2 billion project to replace the roadway with a deep-bore tunnel.

The state hung a banner thanking the structure for its years of service. The waterfront part was finished in 1953 and the southern mile — now being torn down — in 1959.

Commuters will find out Monday whether fears of snarled traffic, or “Viadoom,” will come to pass as the city loses one of its two primary north-south routes. About 90,000 vehicle trips per weekday must disappear or detour from the viaduct, which is scheduled to remain closed until 5 a.m. Oct. 31.

Traffic psychology

Transportation sages are becoming optimistic the public will adapt by staying home, driving at different times of day, or switching to public transit.

Modeling by Kirkland-based INRIX, a leading traffic-data company, suggests a potential regional drop in traffic of around 20 percent, at first.

“Don’t believe the hype, at least not for Monday or Tuesday, but maybe at the end of the week,” Jim Bak, INRIX community-relations director, said about the gridlock predictions.

In August 2007, when three lanes of Interstate 5 closed for repairs, peak trips dropped about 30 percent the first day — a good benchmark for what it would take to keep traffic flowing through Seattle this week.

Thursday is usually the busiest traffic day, so there could be heavy loads by then as people see that congestion isn’t so bad and then return to the roads, Bak said.

Peter Hahn, director of Seattle Department of Transportation, shared that view.

“You try and figure out mass psychology of thousands of people. Who knows what they’re going to do?” said Hahn.

Matt Preedy, Sodo projects director for the state Department of Transportation (DOT), said things will be fine as long as motorists don’t assume their fellow travelers will stay off the roads.

“I personally think people are going to adjust,” he said. “The communication’s been going well; people are picking it up.”

The viaduct closure didn’t cause unusually large backups Saturday, except around First Avenue South along the work zone, where fans streaming in for the Cougar football game at CenturyLink Field mixed with viaduct visitors.

Southbound I-5 from downtown to Northgate was stop-and-go at midday due to an accident.

To help alleviate traffic congestion Monday, transportation managers have added water-taxi parking spots and trips from West Seattle, 30 extra buses to serve Westside routes, traffic police at Sodo boulevards and the waterfront trucking routes, and extra park-and-ride space at the Tukwila International Boulevard light-rail station.

But those measures alone won’t make up for the lost road lanes.

Bak and others encourage drivers to look at online traffic-flow maps before heading out.

Kathryn Seymour, a loan officer who drives from Ballard to her office in Sodo, said if the maps on Monday show gridlock on I-5, “I’m going to work from home!”

But if traffic is moving, she’ll hop in the car.

A last trip on foot

For a few short hours on the viaduct Saturday, it was a time not to worry but to reminisce.

Allison Green, of Bellevue, was among the last people to step off the highway as the walkabout ended.

She said her father made reinforcing steel for the viaduct at the Bethlehem Steel Mill in West Seattle. Green says she drove her 90-year-old mother, Roberta, back and forth on the highway before it closed Friday night.

On Saturday, two men played Frisbee on the deck, a woman wearing cat ears hula-hooped, and children dipped their fingers and shoes into the 3-inch expansion-joint gaps between road decks.

A woman who goes by the name Turtle asked project director Preedy for a prying tool so she could pull up one of the old lane markers — also known as “turtles” — from the deck. Another woman yanked away loose pebbles using her fingers.

Families stood on concrete dividers to be photographed, a football-field’s length away from where a huge aerial hammer kept pounding. The deck trembled with each blow.

As Noel Povlsen noted, the demolition marks the end of any legal threat that might keep at least that part of the structure standing.

“It’s now officially injunction-proof!” said Povlsen, standing with his 8-year-old son, Thor, while the machine broke through guardrails.

Damage to the highway by the Nisqually earthquake a decade ago set off years of costly debate over whether to replace it with another viaduct, a tunnel or a surface route — or shore up the columns and leave it standing.

Some visitors went home thinking a segment of the old viaduct should be preserved as a historical park, similar to the High Line in New York City, where an elevated rail line is now a park.

“To me, it’s the ultimate democratic view, when we’re moving toward plutocracy,” Povlsen said.

Park idea unlikely

Gov. Chris Gregoire, on hand to mark the onset of demolition, said the state has no plans or money to preserve any portion of the viaduct after the central part is removed in 2016, and future parks are the city of Seattle’s domain.

Earlier Saturday, the Rat City Rollergirls and the Cossacks motorcycle-stunt team had the top deck north of stadiums to themselves for a half-hour, the prize for winning a contest sponsored by the DOT.

After the nine-day closure, the waterfront viaduct segment will be open for traffic for another four years, until a deep-bore tunnel and stadium interchange are completed by the start of 2016. Then that stretch of viaduct, too, will come down.

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