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).
Puget Sound Gateway Program: Combining SR 509 and SR 167
In 2015, the Washington State Legislature funded the SR 509 Completion Project in King County and SR 167 Completion Project in Pierce County jointly, under the Puget Sound Gateway Program. Combining both of these large projects helps maximize efficiencies in planning, environmental review, design and construction.
SR 509 Completion 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 Seattle-Tacoma International 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 SEA Airport.
- Intelligent transportation. Electronic tolling on the SR 509 Expressway will manage congestion, which improves safety and reliability for all users.
- Advanced wetland mitigation. Improves water quality and wildlife habitat by completing environmental improvements in the project area.
Three construction stages for the SR 509 Completion Project (2020-2029)
Stage 1a - Open to traffic
For Stage 1a of the SR 509 Completion Project, WSDOT partnered with Sound Transit’s Federal Way Link Extension project to build a new SR 99 bridge near South 208th Street and new walls to reduce noise to nearby neighborhoods. The partnership reduces construction impacts by building both projects at the same time. The SR 99 bridge was opened to traffic in July 2022.
Stage 1b - In progress
Stage 1b of the SR 509 Completion Project, called the SR 509/I-5 to 24th Avenue South – New Expressway Project, builds the first mile of new SR 509 Expressway, new I-5 ramps, new interchanges and a new bridge. Project visualizations are available on our Flickr page and WSDOT's Fall 2021 online open house. This section of the new tolled expressway is open to traffic with some construction continuing into 2026.
Stage 2 - 2024-2029
Stage 2 of the SR 509 Completion Project, called the SR 509/24th Avenue South to South 188th Street – New Expressway Project, is the project's final stage. In Stage 2, WSDOT will build the remaining 2 miles of new SR 509 Expressway, reconfigure SR 509 interchanges at South 188th and South 160th Streets and add a southbound auxiliary lane on I-5 between SR 516 and South 272nd Street. Construction work began in late 2024 and this section of the new expressway is scheduled to open to traffic in 2028, with some construction work continuing into 2029.
