As roads start to close in Seattle on Jan. 4, ahead of the Jan. 11 full shutdown of the Alaskan Way Viaduct, the shuttering of the infrastructure carrying what is known as State Route 99 leads to a long-sought end: traffic through a bored downtown tunnel that allows for the removal of the viaduct.

But getting from current viaduct to newly finished tunnel, which opens in early February, requires a three-week period in downtown Seattle that doesn’t leave either as an option. It will be the longest highway closure the Puget Sound region has ever experienced, with more than 90,000 vehicles traveling the viaduct each day now needing to scatter to limited options.

“The opening of the SR 99 tunnel will be an historic event in the state’s transportation history,” says Brian Nielsen, administrator of the Alaskan Way Viaduct Replacement Program for the Washington State Dept. of Transportation (WSDOT), in a statement. “Before we can celebrate, we have to get through an unprecedented closure that will require all of us to change our behavior.”

The plan involves closing the off-ramp from southbound SR 99 to South Atlantic Street on Friday, Jan. 4, and then closing the entire highway through Seattle on Friday, Jan. 11. Closing SR 99 through Seattle is the only way crews can finish building the highway and the eight new ramps that will allow travelers to enter and exit the new tunnel.

Drivers should plan for about six weeks of regional congestion as crews complete final connections to and from the new tunnel, WSDOT says. Along with the major highway closing for three weeks and the additional one week needed on the South Atlantic Street off-ramp, a new off-ramp from northbound SR 99 to South Dearborn Street will require up to two weeks of additional work after the tunnel opens.

“We need drivers to change their habits for three weeks to prevent gridlock,” Nielsen says. “We recognize everyone’s strategies will be different based on their needs, but consider other ways to get to and from your destination, if you can.”

The last time the viaduct closed for a substantial portion of time—from April 29 to May 8 in 2016—WSDOT says commutes started earlier and lasted longer, even on Seattle city streets. While officials did see a 130 percent jump in riders on the King County Water Taxi to and from West Seattle, that didn’t diminish the more difficult commute for nearly everyone in the Seattle region.

When Seattle Tunnel Partners, the contractor on the multibillion-dollar project, finished disassembling tunneling machine Bertha in 2017, WSDOT estimated the tunnel would open in early 2019. While there was optimism early in 2018 that this date could be moved up, the decision was made to keep with the original expectations to give several contractors time to complete weather-dependent work preparing for the three-week closure.

Setting the road closures in January with a tunnel opening in February not only ensured pre-closure work could wrap up, but gave WSDOT additional time to alert the public to the closure plans. Plus, in working with the Seattle Dept. of Transportation, King County Metro and other transportation agencies, WSDOT decided to avoid major highway closures between Thanksgiving Day and New Year’s Day.

When initially opened, the tunnel will be free to use, but will eventually become tolled as part of the project’s financing plan.

Follow Tim Newcomb on Twitter at @tdnewcomb.