SR 509 - Completion Project - SR 99 Bridge Construction

Welcome to the SR 509/SR 99 Bridge construction page! We will update this page regularly as construction progresses.

January 2021

  • Our SR 509/I-5 to 24th Avenue South Expressway Project pre-construction online open house is now available. The online open house provides opportunities to learn more about the next stage of construction for the SR 509 Completion Project in south King County. The project includes significant construction along and near I-5 in Des Moines, Kent, and SeaTac.  Guy F. Atkinson Construction, our design-build contractor, is starting a year-long final design phase for the project. Construction is expected to start in 2022 and wrap up in 2025.  The online open house is available until March 5, 2021 and provides opportunities to comment before final design is complete. The online open house is also available in simplified Chinese, Spanish, and Vietnamese. 
  • In partnership with WSDOT, Sound Transit is currently building a new SR 99 bridge over SR 509 near South 208th Street in SeaTac. The work is part of the Federal Way Link Extension (FWLE). To make room for construction, there is a temporary bypass now in place on SR 99 between S. 204th and S. 208th streets in SeaTac. The bypass will stay in place until bridge construction is completed, scheduled for October 2022.

Updated December 2020

  • In December 2020, WSDOT awarded the “apparent best value” contract for the SR 509/I-5 to 24th Avenue South – New Expressway Project (formerly Stage 1b).  This portion of construction is large and complex and the design-build contractor will be in the design phase for much of 2021. Construction is estimated to start by early 2022.
  • Sound Transit is building a temporary bypass around a short stretch of State Route 99 in SeaTac between South 204th and South 208th Streets in order the construct the SR 99 bridge for the SR 509 1a Project. Drivers will start using the bypass in January and won't return to the normal SR 99 configuration until October 2022. 

Updated October 2020

  • Earlier this year SR 509 Completion Project entered Stage 1a, which will construct a new SR 99 bridge over SR 509 near South 208th Street in SeaTac. Sound Transit is constructing this bridge on behalf of WSDOT as a part of the Federal Way Link Extension (FWLE).
  • Detour maps and additional information on upcoming work can be found on the FWLE News and Updates page.

A rendering of the future SR 99 bridge and Sound Transit Link light rail over SR 99