Project overview
The SR 509 Completion Project in King County is part of the Puget Sound Gateway Program, which also includes the SR 167 Completion Project in Pierce County. Together, these projects complete two of the Puget Sound region’s most critical freight corridors and improve access to I-5, the ports of Tacoma and Seattle and Seattle-Tacoma International Airport. The SR 509 Completion Project builds a new four-lane expressway between I-5 and SR 509's current end near the airport, builds new I-5 ramps, improves I-5 interchanges in south King County and constructs new bridges. When complete, the project will create an important north-south alternative to the congested I-5 corridor between Seattle and south King County.
What to expect
Eastbound South 188th Street to SR 509 ramps closed: new ramp opened
SR 509 ramps to and from South 188th Street/Des Moines Memorial Drive South will changed in August 2025 for approximately two years. Drivers are using a new traffic signal when entering northbound SR 509 from South 188th Street/Des Moines Memorial Drive South or exiting southbound SR 509 to South 188th Street/Des Moines Memorial Drive South. The traffic shift is necessary to create a work zone for the completion of the new SR 509 Expressway and to prepare for improvements at this interchange later in the project.
Temporary closure of a portion of the Lake to Sound Trail near the SR 509/South 188th Street interchange
Approximately a half-mile of the Lake to Sound Trail is closed through November 2025. The closed portion of the trail is between its connections with westbound Des Moines Memorial Drive South near Eighth Avenue South and southbound Des Moines Memorial Drive South near South 190th Street. People using the trail will be detoured to the sidewalks on the west and south sides of Des Moines Memorial Drive South. This closure is necessary to keep travelers and construction crews safe during work on the new State Route 509 corridor at the South 188th Street interchange. Flaggers will be in the area to assist with safe crossings as needed.
The SR 509 Completion Project builds three new miles of SR 509 and completes the unfinished SR 509 in south King County. This new four-lane highway will become an important north-south alternative to the congested I-5 corridor between Seattle and south King County. The new segment of SR 509 will be called an expressway and will be tolled at one electronic toll point (no tollbooths).
