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SR 509 Expressway

This page describes project key features. The designs presented are conceptual. The design-builder will finalize schedules and designs, which may vary from the conceptual designs.

SR 509 Expressway

The final two miles of the new four-lane SR 509 expressway extend from 24th Avenue South to South 188th Street in SeaTac, where the new expressway meets up with the existing SR 509 highway.

Aerial view of project Stage 1b and Stage 2
The SR 509/24th Avenue South Interchange will open in 2025 upon completion of the first mile of the SR 509 expressway under Stage 1b of the SR 509 Completion Project.
An overhead view of the new 24th ave bridge
The 24th Avenue South interchange will include a southbound SR 509 on-ramp and northbound off-ramp leading to 24th Avenue South.

To maintain access to local roadways in the area, the expressway will cross over South 200th Street and Des Moines Memorial Drive and under a new bridge at South 192nd Street. In Des Moines, South 194th Street will no longer serve as a direct connection between 8th Avenue South and Des Moines Memorial Drive. Local access to Des Moines Memorial Drive from South 194th Street will be maintained via 11th Place South and South 196th Place. South 196th Street and 18th Avenue South in SeaTac will no longer serve motorized traffic, instead serving as a critical portion of a new segment of the Lake to Sound multi-use trail, funded by the project.

Renderings of 192nd St bridge and interchange
A 2018 environmental (NEPA) re-evaluation confirmed that updated designs comply with federal and statewide standards on environmental health, environmental protection, and environmental justice, including ensuring that the project maintains sufficient local, transit, and multi-modal access.


To manage traffic demand and generate funding for project construction and ongoing maintenance costs, in 2019 the Washington State Legislature authorized electronic tolling on the new SR 509 expressway. Only the new section of SR 509 (between South 188th Street and I-5) will be tolled; all existing sections of the SR 509 highway will remain untolled.

There will be one toll point (no tollbooth) on the SR 509 expressway located between I-5 and SR 99. Tolling the new SR 509 expressway will enable WSDOT to better manage traffic volumes in the corridor. Providing this express connection to regional ports means freight traffic will be far less likely to use local roads, easing congestion and reducing emissions from excess idling.

Toll rates will vary by time of day and will be set by the Washington State Transportation Commission following a public process scheduled to begin in 2024.