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.
Timeline
2015-2029
Project status
Construction
Funding
$2.69 billion funds both the SR 509 Completion Project in King County and the SR 167 Completion Project in Pierce County.
Come to our open house on Feb. 11 about Stage 2 construction
We're hosting an open house in February focusing on upcoming construction work between 24th Avenue South in SeaTac and South 188th Street near the airport, including improvement at the South 160th Street intersection in Burien.
6-8 p.m. Tuesday, Feb. 11
Burien Community Center, 14700 Sixth Ave. SW
People are invited to stop by at any time. WSDOT and contractor staff will be available to answer questions about planned improvements, new roundabouts, anticipated closures, noise, environmental processes and studies, traffic and the toll rate setting process.
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 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 Sea-Tac 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)
This map shows the route of the SR 509 Expressway with the construction stages outlined in different colors.
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 pageand WSDOT's Fall 2021 online open house. This section of the new tolled expressway is expected to open to traffic in 2025, 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. Project visualizations are available on our Flickr page,WSDOT's Spring 2022 online open house, and an interactive map. 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.
2020-2029: Sequenced construction to complete SR 509 in south King County
Milestone
Winter 2020
Construction begins on stage 1a of the SR 509 Completion Project
November 2021
Work begins on stage 1b of the SR 509 Completion Project: The SR 509/I-5 to 24th Avenue South–New Expressway Project
Winter 2022
Substantial completion of stage 1a of the SR 509 Completion Project
Fall 2024
Start of stage 2 (final stage) of the SR 509 Completion Project, which builds the final 2 miles of new SR 509 Expressway
2025
The first mile of SR 509 Expressway, which will be tolled, opens to traffic.
2028
Expected opening of the final 2 miles of the expressway.
2029
Expected completion of SR 509.
The Puget Sound Gateway Program has $2.69 billion in funding from four sources:
Stage 1a built a new SR 99 bridge over the future SR 509 Expressway. Light rail will travel above. Sound Transit built the SR 99 bridge for WSDOT as part of the Federal Way Link Extension.
This visualization shows the first new mile of SR 509 Expressway, connecting ramps to I-5 and a new South 216th Street bridge over I-5 that will also includes sidewalks and bike paths on both sides.
This visualization shows local road improvements in SeaTac's Madrona neighborhood, including a new 34th Avenue South which connects South 208th Street to South 204th Street. There is also a new South 206th Street that will connect South 208th Street to SR 99.
This visualization shows a rebuilt I-5/SR 516 (Kent-Des Moines Road) interchange along with an extended Veterans Drive, which will improve traffic flow in the Kent area.
The new SR 509 Expressway will be built in two stages. This map shows the planned route for the SR 509 corridor from I-5 to the current end of SR 509 at the southwest corner of Sea-Tac Airport.
SR 509 online open houses
The online open house outlining the second stage of construction shows work planned for SeaTac, Burien, Kent and Des Moines is now available. Visitors can check out work by different areas and neighborhoods. The comment period for this open house has closed.
WSDOT's spring 2022 online open house presented conceptual designs for Stage 2 of the SR 509 Completion Project and includes the public release of the SR 509 Completion Project introduction video. The open house is available in Arabic, Simplified Chinese, English, Somali, Spanish, Tagalog, and Vietnamese. The comment period for this open house has closed.
WSDOT's fall 2021 online open house presented designs and potential construction impacts of Stage 1b of the SR 509 Completion Project. The open house is available in Arabic, Simplified Chinese, English, Somali, Spanish, Tagalog, and Vietnamese. The comment period for this open house has closed.
