The long road to opening the State Route 99 tunnel under downtown Seattle finally has an official end in sight.

The project's owner, the Washington State Dept. of Transportation, announced plans to start a series of road closures on Jan. 4, 2019, that will lead to the opening of the tunnel to traffic sometime around the start of February 2019. This is the first official announcement of concrete dates on the opening of the new tunnel.

Even without locking in a final opening date, WSDOT says to plan for the new tunnel to open about three weeks following the full closure of State Route 99, which happens on Jan. 11.

Opening the tunnel is about more than simply removing a few barriers. State transportation crews must first realign the state highway and then move SR 99 from the seismically vulnerable Alaskan Way Viaduct into the tunnel. This work will last about three weeks and prompt the longest major highway closure to ever hit the Puget Sound region.

“The opening of the SR 99 tunnel will be an historic event in the state’s transpiration history,” says Brian Nielsen, administrator of the Alaskan Way Viaduct Replacement Program, 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 current plan calls for 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.

WSDOT says drivers should plan for about six weeks of regionwide congestions as crews complete final connections to and from the new tunnel. 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 about the 90,000 drivers who daily use the viaduct in downtown Seattle. “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.”

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 this date could be moved up, the decision was made to keep with the original expectations. WSDOT says they didn’t want to move up the timing because several contractors must still complete work to be ready for the three-week closure, including Scarsella Brothers Inc. building the final tunnel and ramp connections. This work comes weather dependent.

Starting closures in January ensures that contractor work will have wrapped up and WSDOT can provide specific date information much further in advances. 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 originally 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