SR 509 Completion Project

Project news

  • In December 2020, the SR 509 Completion Project marked a major milestone with the apparent best value contract award for the SR 509/I-5 to 24th Avenue South – New Expressway Project (formerly Stage 1b). This stage of construction builds the first mile of new SR 509 Expressway along with significant improvements from Kent to SeaTac. Learn more about the work here.
  • WSDOT joined with Sound Transit to build the first stage of the SR 509 Completion Project within the Federal Way Link Extension project. The partnership reduces the impacts of major construction by building both projects at the same time.
  • As part of this project, Sound Transit is building a new SR 99 bridge near South 208th Street on WSDOT’s behalf. The new SR 509 Expressway will travel under the bridge while light rail will travel over the bridge. As part of the project, Sound Transit will build a new wall to help reduce noise from both the new roadway and light rail.  Construction is now underway and is targeted for completion in 2022.

Needs & benefits

The SR 509 Completion Project extends SR 509 to I-5 near Tukwila, adds a southern access point near Sea-Tac International Airport, and improves service between industrial districts by allowing general purpose traffic and trucks to bypass I-5, SR 99 and local streets.

When finished, SR 509 will be a key component of the Seattle and south King County transportation network. Along with the SR 99 tunnel improvements, the project provides a critical north-south alternative to I-5 through Seattle and King County.

Check out our video about the SR 509 Completion Project to learn about the project’s benefits and get a glimpse of what it will look like when it’s completed.

Project benefits

  • Freight connections. Completes critical freight links between the Port of Seattle and key distribution, warehouse and industrial centers in King County.
  • Regional mobility. Reduces traffic congestion on local roads and highways by completing connections between Seattle and the Kent Valley. Provides an alternate route to I-5 in south King County. 
  • Airport Access. Provides a new connection between I-5 and Sea-Tac Airport from the south for both passengers and air cargo.
  • Regional job and economic growth. Supports regional job growth and economic growth associated with the state's two largest seaports and airport.
  • Intelligent transportation. Electronic tolling on the route will manage congestion, which improves safety and reliability for all users.
  • Advance wetland mitigation. Improves water quality and wildlife habitat by completing environmental improvements in advance of the project.

Construction partnerships

  • We have contracted with King County to fund 1.8 miles the Lake to Sound Trail in SeaTac and Burien. Once completed, the trail will provide a 16-mile non-motorized, multiuse connection between Lake Washington in Renton and the Puget Sound in Des Moines.
  • As part of our construction agreement with Sound Transit, their Federal Way Link Extension contractor will build Stage 1a of the SR 509 Completion Project for us. This arrangement will reduce community impacts and help keep us on schedule. Sound Transit is scheduled to begin bridge construction later in  2020.


The SR 509 Completion Project was originally funded over a 16-year timeline with targeted completion in 2031. The state Legislature advanced Program funding in the 2019 session, moving project completion to 2028.

  • Stage 1a: Constructs a new bridge over SR 99 near South 208th Street; This work will be done by Sound Transit and will take about two and a half years to complete.
  • SR 509/I-5 to 24th Avenue South – New Expressway Project (formerly Stage 1b):
    • Reconstructs the I-5/SR 516 interchange
    • Builds new SR 509 ramps to and from I-5
    • Constructs a new interchange at 24th Avenue
    • Builds a new South 216th Street I-5 overpass. This new overpass provides pedestrian and bike access, which does not exist on the current 216th Street bridge.
  • SR 509/24th Avenue South to South 188th Street – New Expressway Project (formerly Stage 2):  
    • Constructs the majority of the new SR 509
    • Adds a southbound auxiliary lane on I-5 between SR 516 and South 272nd Street between 2024 and 2028.


Gateway Program funding totals approximately $2 billion from four sources: 1) $1.6 billion from the Connecting Washington funding package; 2) $130 million from local contributions; 3) $180 million from tolling; and 4) Federal grants. The SR 509 Completion Project will receive about $1 billion of the Program’s total funds.


Media Inquiries
Laura Newborn

Construction Inquiries
Chris Hoffman

SR 509 completion project map