• Project

SR 509 Completion Project

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 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.

Project status
Estimated $2 billion funds the Gateway Program, which includes the SR 509 Completion Project and the SR 167 Completion Project
Project hotline

What to expect

When construction starts in November 2021, drivers on I-5 can expect southbound lanes of I-5 in the vicinity of the SR 516 interchange in Kent to shift towards the median as crews begin work to widen the I-5 bridge over SR 516.

In early 2022, work on I-5 starts at the South 216th Street bridge where Des Moines and SeaTac meet. Travelers should expect lane shifts and nighttime lane closures in the area.

In SeaTac, a temporary bypass on SR 99 between South 204th and South 208th streets remains in place until late 2022, when a new SR 99 bridge over the future SR 509 Expressway will be completed.   


The SR 509 Completion Project completes 3 miles of the unfinished SR 509 to I-5, making SR 509 a key component of the Seattle and south King County transportation network. The new road will be called an expressway and will be tolled at one electronic toll point (no tollbooths).