SR 509

Route ID
509

WSDOT opens section of new SR 509 Expressway in SeaTac June 28

The first mile of the expressway extends from I-5 to South 24th Street

SEATAC – The first section of a new expressway is about to open in the Puget Sound area, providing travelers with another commuting option in south King County.

At 5:09 a.m. on Saturday, June 28, the Washington State Department of Transportation will open the first mile of the State Route 509 Expressway. The first mile of the SR 509 Completion Project stretches between Interstate 5 and 24th Avenue South in SeaTac. 

A celebration of connections marks the final stage of SR 509 construction in South King County

SR 509 Completion Project will connect the 50-year-old highway to I-5

SEATAC – Community, industry and elected leaders joined together in SeaTac on Wednesday, Sept. 25, to celebrate the beginning of construction on the final stage of the State Route 509 Completion Project in south King County. 

File field should be

1,557 work zone-related crashes

were recorded on state highways in 2025.

542 people were injured

in 2025 due to work zone-related crashes. 

29% increase in fatality collisions

in work zones from 2024 to 2025.