The Washington State Department of Transportation (WSDOT) has awarded a construction contract to finish the final stage of the State Route 509 extension project that will impact Kent, SeaTac and Des Moines.

The first stage is scheduled to open in 2025 and the second stage in 2028.

Guy F. Atkinson Construction, whose northwest office is in Renton, was awarded the contract for $478.8 million, less than 1% over the state engineer’s estimate of $475.4 million, according to a Dec. 7 WSDOT news release.

Stage 2 builds the remaining 2 miles of the tolled expressway, from 24th Avenue South to South 188th Street, that will complete the connection between Interstate 5 and SR 509’s current end near Sea-Tac Airport.

“Finishing SR 509 will improve freight mobility to and from our ports and provide an important north-south alternative to I-5 between Seattle and south King County,” said Gateway program administrator John White. “Atkinson Construction has a long history of successfully delivering complex highway projects for WSDOT. We’re looking forward to working with them to complete the design and construction of the final 2 miles of SR 509.”

Stage 2 construction

The SR 509 Stage 2 project includes two lanes in each direction of the future expressway. It rebuilds the interchanges at South 160th Street on the existing portion of SR 509 in Burien and South 188th Street in SeaTac, where SR 509 currently ends.

It also builds a southbound I-5 auxiliary lane between SR 516 and South 272nd Street in Kent. In Des Moines, the project will improve 14 acres of land near Barnes Creek off Des Moines Memorial Drive.

Stage 2 is a design-build contract, which combines project final design and construction into a single contract, according to the news release. Atkinson Construction will spend most of 2024 completing design and planning work. Construction is expected to begin in late 2024 or early 2025, with the new expressway scheduled to open to traffic in 2028.

State 1B work

Stage 1b of the SR 509 Completion Project is underway after beginning construction in 2021. It builds the first mile of the new expressway between I-5 and 24th Avenue South. Stage 1b also includes a northbound I-5 flyover ramp to SR 509, a reconfigured I-5/SR 516 interchange, a new Veterans Drive tunnel under the interstate and a new South 216th Street bridge. This stage of construction is expected to be complete in 2025.

Full project

The SR 509 Completion Project builds a new four-lane, 3-mile-long tolled expressway. Completing this long-planned connection will help reduce congestion on local roads and highways by providing more direct access to Sea-Tac Airport from the south for both passengers and air cargo and the Port of Seattle for marine shipping.

The SR 509 Completion Project is part of the Puget Sound Gateway Program, which also includes the SR 167 Completion Project in Pierce County. Together, these two projects will complete critical missing links in Washington’s highway and freight network, according to WSDOT. Both projects have multimodal benefits and create more than 18 miles of sidewalks, bike lanes and shared-use paths.

Dark, new asphalt clearly shows the path of a new on-ramp from the southbound SR 509 Expressway, still under construction, to southbound I-5 in the Kent, Des Moines, and SeaTac areas. COURTESY PHOTO, WSDOT