SR 509/24th Avenue South to South 188th Street New Expressway – What is this project?

A map showing the SR 509 Completion Project area. The SR 509 Completion project extends from South 160th Street in Burien to South 272nd Street in Kent. Stage 1b of the project extends from 24th Avenue South in SeaTac to SR 516 in Des Moines. Stage 2 of the project sandwiches Stage 1b, extending north from 24th Avenue South to South 160th Street and extending South from SR 516 to South 272nd Street. This map also highlights a wetland mitigation site east of the project corridor at Barnes Creek in Des Moines.

Project overview

The SR 509/24th Avenue South to South 188th Street New Expressway project is the final stage of the SR 509 Completion Project improvements, which connects SR 509 to I-5 in SeaTac. Once the project is complete in 2028, people traveling through the area will be able to bypass SR 518 and local streets when traveling between I-5, Sea-Tac Airport, the Port of Seattle and population centers in south King County. Community members will experience fewer container trucks on local roads as freight drivers will have a new route to reach their destinations. The new portion of SR 509 will be tolled. Throughout construction and after completion, people will still be able to use their current access points to SR 509 and I-5, and people will not pay tolls on the roads they use today.

What’s included in the project?

The SR 509/24th Avenue South to South 188th Street New Expressway project includes:

Over two new miles of expressway.
An additional lane on southbound I-5 between SR 516 and South 272nd Street in Kent.
Roundabouts at South 160th Street in Burien and South 188th Street in SeaTac.
New noise walls.
Restoring natural areas within the project corridor.

Project timeline

Early construction activities include vegetation clearing throughout the project area. Heavier construction work will begin in fall 2024. Check out the work zones in the following pages to learn what to expect in the areas you live, work and travel.

A timeline displaying the SR 509 project stages from start to finish. Stage 2 construction begins in 2024 and will be open to traffic in 2028.

The SR 509 Completion Project is already under construction. Stage 1a opened to traffic in 2022, Stage 1b is expected to open to traffic in 2025, and Stage 2 is expected to be open to traffic in 2028.