Friday evening travelers should plan ahead as South 200th Street in SeaTac will close overnight Friday, May 8 to the morning of Saturday, May 9, according to the Washington State Department of Transportation (WSDOT).

This closure will allow WSDOT contractor crews to set the last of 16 girders for a 1,587-feet-long bridge.

This massive bridge will carry the new State Route 509 Expressway over the Des Moines Creek wetland, South 200th Street and the Lake to Sound Trail.

This work is part of the SR 509 Completion Project.

South 200th Street closure information

Friday, May 8 to the morning of Saturday, May 9:

  • 8 p.m. to 5 a.m. – South 200th Street will close overnight in both directions between 18th Avenue South and Des Moines Creek Trail. A signed detour will be in place.

About the girders

Bridge girders are horizontal beams that support the bridge’s weight and form the foundation of the bridge deck. Spanning between piers, these girders will carry the new structure over South 200th Street below. The girders spanning South 200th Street range in length from 160 to 164 feet. If they were stacked atop each other, they would be taller than four Space Needles.

SR 509 Completion Project information

The SR 509 Completion Project builds 3 miles of new tolled highway between I-5 in SeaTac and South 188th Street near the south end of Seattle-Tacoma International Airport. Crews are building the new expressway in stages. The first stage, completed in 2022, built a new bridge on SR 99 in SeaTac. The next stage, which opened in summer 2025, built the first mile between I-5 and 24th Avenue South. That stage also included the new Veterans Drive tunnel in Kent near the SR 516 interchange.

Crews are now working on the final stage which builds the remaining 2 miles of the expressway between 24th Avenue South and South 188th Street in SeaTac. The entire expressway is scheduled to open to the public in 2028.

Puget Sound Gateway Program overview

The SR 509 Completion Project is part of WSDOT’s Puget Sound Gateway Program, which also includes the SR 167 Completion Project in Pierce County. Together, the two projects complete critical missing links in Washington’s highway and freight network.