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 Sea-Tac 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
Construction work near Des Moines Creek and Lake to Sound trails
Construction work around the Des Moines Creek and Lake to Sound trails began in March 2025. Work is expected to last through 2028. Spring 2025 work includes cutting down and removing trees along the trails. In summer 2025, crews will begin earthwork along the SR 509 corridor. This will require large construction vehicles, including trucks with trailers, near or frequently crossing the trails. Trail users will see advance warning signs. At times, flaggers may also be on site to direct trail users and construction vehicles.
Eastbound SR 516 ramp to northbound I-5 closed; detour provided
The eastbound SR 516 ramp to northbound I-5 is closed until early summer 2025. While the ramp is closed, drivers should stay in the left lane of eastbound SR 516 to enter a temporary left turn lane and temporary traffic signal to enter northbound I-5. The turn lane and traffic signal are located just east of the I-5 overpass. The existing loop ramp is being rebuilt to align it with an intersection where other I-5 on- and off-ramps are located and remove some of the sharp curve of the ramp.
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).
