The Washington State Department of Transportation (WSDOT) is progressing with the SR 509 Completion Project in King County, part of the Puget Sound Gateway Program. A key segment of the project—the first mile of the new four-lane SR 509 Expressway from I-5 to 24th Avenue South—is expected to open to traffic later this year. This section, known as Stage 1b, includes new interchanges, I-5 ramps, and a new bridge. Some construction will continue into 2026.
The broader SR 509 Completion Project will build a direct connection between I-5 and the current end of SR 509 near Seattle-Tacoma International Airport. Once complete, the project will offer a north-south alternative to I-5, improve freight mobility, and enhance access to the airport, ports, and industrial centers across the region.
The multi-phase project includes:
- Stage 1a (Completed July 2022): SR 99 bridge and noise walls near South 208th Street.
- Stage 1b (In Progress): New expressway from I-5 to 24th Avenue South, expected to open in 2025.
- Stage 2 (2024–2029): Two additional miles of expressway, interchange reconfigurations at South 160th and South 188th Streets, and an auxiliary lane on I-5 between SR 516 and South 272nd Street.
The project is funded through a $2.83 billion package that includes:
- $2.44 billion in state funds, including allocations from Connecting Washington and Move Ahead Washington.
- $143.9 million in local contributions.
- $213.2 million in toll revenue.
- $74 million in federal INFRA grant funding.
Once operational, the SR 509 Expressway will feature electronic tolling to manage congestion and improve travel reliability.