In a few days, we learn whether Seattle will screech to a halt without its Alaskan Way Viaduct.

The 1950s-vintage elevated highway, the star of car commercials and the bane of mayors, will close from 7:30 p.m. Friday to 5 a.m. Oct. 31, so its southern mile can be demolished as part of a project to eventually replace it with a tunnel.

Even on a normal day, the city's road network is stretched so tight a single crash can cause radiating backups in three or more directions. About 90,000 vehicles a day must detour or disappear next week.

But transportation providers are making only marginal tweaks for the week — so they're relying mainly on the adaptive powers of you, the commuter.

"If nobody changes how they operate, nothing will move," says Matt Preedy, the Sodo-area construction director for the state Department of Transportation (DOT). "If the majority of people change the way they operate, things may not be so bad."

Just how backed up things might get, he won't guess.

In August 2007, the state hyped a potential backup of 10 to 30 miles when three lanes of I-5 were closed for expansion-joint repairs, but the actual queue averaged three miles, as people took days off or switched to transit.

However, on March 1, 2001, after the Nisqually earthquake, an inspection closure of the viaduct increased crosstown trips to nearly three hours. And on Nov. 21, 2010, a sudden afternoon freeze outwitted local agencies, and many commutes exceeded five hours.

What's your ride?

If you're changing trip plans to avoid Viadoom, the best advice is to try a new route Friday. You may be amazed at how complicated it is to buy a train ticket for the first time, bicycle through the waterfront trucking corridors, or drive crosstown from White Center to Beacon Hill.

Here are the prospects for various modes of travel:

Bus

King County Metro has the most aggressive strategy, bringing 30 extra buses on duty to reduce wait times and carry overflow passengers. An Oct. 1 service change increased midday trips on West Seattle's workhorse Route 54.

Planners devised a clever strategy to get commuters downtown in the morning: Buses that normally take the viaduct will exit their bus lane at First Avenue South to lightly used lower Spokane Street, then be waved by a traffic cop to a bus-only left turn at Fourth Avenue South. That road has plenty of room. But if it gets clogged, riders can get off and walk a block to meet Link light rail at Sodo Station, to the downtown transit tunnel.

Seattle just added a morning bus lane on Avalon Way Southwest in West Seattle, but it covers only a few blocks, so buses might still be snared farther uphill.

It's in the evening when Metro is defenseless. Lacking a viaduct junction to the high West Seattle Bridge, those buses will wend through Sodo to the lower swing bridge. The Coast Guard agreed to avoid nearly all maritime openings from 3 p.m. to 6 p.m., but train crossings by BNSF Railway and heavy truck traffic to the Port of Seattle are likely. Four cargo ships are coming in Sunday, and three more are coming in Wednesday.

Northsiders can expect delays because the Battery Street Tunnel will close and cause congestion on Aurora Avenue North, including Route 358, and because some lines serving Greenwood, Shoreline and Ballard use buses arriving from West Seattle, which will take longer to reach the north.

Car

West Seattle Bridge traffic will be slow. Drivers can get off at First Avenue South — where the city's one-lane work-zone exit will be opened to two lanes — or try the new Fourth Avenue South loop ramp into Sodo, where the city's traffic lights are well-timed.

The viaduct will be open — northbound only — from the stadium onramp during the daytime on weekdays and during sporting events at CenturyLink Field.

A dilemma for residents farther south is where to cross the Duwamish River.

Interstate 5 does the trick, but Matt Preedy, of the DOT, thinks it will be more jammed than usual. Motorists could exit early to northbound Highway 599 in Tukwila, but that leads to bottlenecks at the First Avenue South bridge.

An alternative is the I-5 exit to Airport Way South, which has room but slows through the resurgent Georgetown business district. The city helped by postponing a seismic project on Airport Way.

Or follow nearby East Marginal Way South to either First or Fourth avenues South. A ways north, the Seattle Transportation Department will suspend ramp construction at First and Spokane so two through lanes will flow in each direction.

From White Center and Burien, a tricky way to avoid the First Avenue South Bridge mess is to begin at South 116th Street and wind downhill to Tukwila International Boulevard — which crosses the river near Boeing Field. Likewise, go from the Southcenter area or exit early from 599 onto Interurban Avenue South, which crosses a short Duwamish bridge.

Park

and ride

Officials are promoting transit, but many of the region's big park-and-ride lots are full. So go early. Sounder and Link transit trains avoid traffic, but those parking lots in Tukwila already fill up. Officials suggest "kiss and ride," having someone drop you off. Some commuters "hide and ride" — that is, park along streets near a bus stop.

Boat

Seattle is opening 65 temporary park-and-ride spots at Don Armeni Boat Launch, and more spaces are on-street, for the water taxi leaving West Seattle. Extra sailings are being added, and the boats are only one-quarter full normally.

Agencies are not adding trips from Vashon Island even though some boats are at their 150-rider capacity already.

Bicycle

Riding might be the quickest way, and better exercise. The downside is watching out for irritated, gridlocked drivers, especially in crowded South Lake Union. "The whole area could be a tricky area for bicyclists during the nine days," said Brian Bothomley, a volunteer bike ambassador with the Cascade Bicycle Club. Novice cyclists can ride with club guides at 7:30 a.m. Monday and Wednesday from the lower West Seattle Bridge to Westlake Park. Or bicyclists can take a trail to the water taxi and roll aboard.

Train

No extra trips are planned by Sound Transit, but dispatchers will be ready to send extra light-rail trains if crowding occurs, said spokesman Bruce Gray. There are private park-and-ride lots in Southeast Seattle near Othello, Columbia City and Mount Baker stations, while on-street parking is restricted within a quarter-mile.

Sounder trains from the north have extra room and could be faster than driving.

Embracing Viadoom

Where some people see crisis, others find opportunities.

Highline Medical Center posted a billboard saying, "We feel your pain," to promote using its Burien hospital and West Seattle clinic instead of trekking through gridlock to First Hill.

Über, a mobile service for people seeking a town car for hire, is giving discounts to people who text "FUBAR" to its website.

The Vintage Park, Hotel Monaco and Alexis Hotel offer room and parking discounts, and free wine tasting, trying to entice people to stay overnight instead of commute.

Not to be outmarketed, Metro is mailing a pair of free-ride tickets to 70,000 households near bus lines in West Seattle, White Center, Sodo, Georgetown and Burien.

When the closure is over, Highway 99 will reopen using a four-lane detour at 40 mph, slowing to an advised 25 mph on a curve near the sports stadiums, until a new Sodo interchange and a tolled downtown tunnel are finished in 2016.

"Every effort helps," Preedy says. "You can help keep traffic moving even if you shift your commute just one day."

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