After three mostly tolerable days of Viadoom traffic, state officials are bracing for a possible lapse on Thursday, unless commuters reduce driving or change trip times.

The Wednesday afternoon commute started early, with stop-and-go traffic heading into Seattle that stretched to Northgate, but not out to North 130th Street as happened Tuesday.

What probably occurred was a bounce-back effect, where people changed plans Wednesday after a nightmarish trip Tuesday afternoon, said Jim Bak of the traffic-data firm INRIX. "We just saw some people who decided to stay on the sidelines," he said.

Drivers heading south out of downtown Wednesday evening experienced long delays in some spots, particularly along waterfront-area streets from Colman Dock through the Sodo area. Traffic was stopped at least three times by BNSF Railway trains, the West Seattle Blog reported. As late as 6:30 p.m., a Seattle Times reporter on a bicycle passed more than a half-mile of stop-and-go traffic trying to leave the waterfront.

Some buses went around a line of traffic heading to the West Seattle swing bridge. A few commuters said on Twitter they were delayed as long as an hour.

Traffic managers continue to recommend Airport Way South as an alternative to crowded highways.

The northbound onramp near First Avenue South, and northbound lanes of the Alaskan Way Viaduct, are now open 24 hours a day, from Royal Brougham Way to South Lake Union, said Jeff Switzer, a state Department of Transportation (DOT) spokesman. Initially, the plan was to have them open only in the daytime or for sporting events.

Bak continues to think Thursday will be the worst commute day, as it normally is the busiest of the week and because motorists might revert to normal patterns.

"We understand drivers can only make adjustments for so long, before going back to their normal routines," said Matt Preedy, DOT Alaskan Way Viaduct replacement program deputy administrator, who warned of possible gridlock.

About 90,000 drivers a day who normally pass the Sodo portion of Highway 99 are being forced to detour, go at different times, stay home or take transit. Demolition of the viaduct's south mile continues on schedule, said Switzer.

A four-lane detour is to open in Sodo by 5 a.m. Monday and last until 2016, when the deep-bored tunnel is to be completed to South Lake Union. The curved detour lanes already have been paved, and are covered by steel plates as debris falls around them.

Mike Lindblom: 206-515-5631