State Route 99 will be closed for three weeks of construction starting Friday, Jan. 11. The closure begins near the West Seattle Bridge and stretches all the way to the southern end of the Battery Street tunnel.

According to the Washington State Department of Transportation (WSDOT), this is the largest highway closure that the Puget Sound region has ever seen. As a result, local residents must plan around high levels of traffic spilling across the greater Seattle metropolitan area until construction finishes in early February.

WSDOT is closing the highway to realign it into a new tunnel parallel to the Alaskan Way Viaduct, with eight new ramps connecting the tunnel to city streets. However, when State Route 99 reopens in February, the viaduct will stay closed until the city begins demolition later in the month.

As a result of the closure, commute times are likely to increase along major routes including Interstate 90 and State Route 520 between Bellevue and Seattle, according to WSDOT studies from the 2016 closure of the viaduct. The same statistics suggest commuters could spend an extra 30 minutes in their cars on Interstate 5 between Federal Way and Seattle, and between Everett and Seattle.

“There are 90,000 vehicles that travel the viaduct every day,” said Laura Newborn, media relations for the Alaskan Way Viaduct Replacement Program. Newborn recommends that Seattle area residents work from home if they can, take different modes of transportation around Seattle, or failing that, carpool.

The Alaskan Way Viaduct was built more than half a century ago. While it survived the 2001 Nisqually earthquake, the tunnel is still vulnerable to another seismic event. The new tunnel will replace the viaduct along with an Alaskan Way surface street on the same stretch as the viaduct.

Seattle commuters have four days left to use SR 99 before it closes.