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
First mile of SR 509 Expressway opens
The first mile of the new SR 509 Expressway in SeaTac is open to travelers. The first mile stretches between I-5 and 24th Avenue South. The first mile features two lanes in each direction. Northbound I-5 drivers will exit to a large fly-over ramp that connects to the expressway. Until the next section of the expressway opens, drivers will exit at 24th Avenue South. Travelers can also enter the expressway from 24th Avenue South and travel southeast to merge onto southbound I-5. The new expressway will be tolled beginning in the fall. Tolling based on time of day will be used to help manage congestion on the new corridor. Toll funding will also help pay for construction and ongoing maintenance.
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 began 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.
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).
