I-5 - SR 161 - SR 18 Triangle Interchange Vicinity - Improvements

Project news

  • Engineering and design work on this project is scheduled to resume in late 2021. 
  • This project received funding through the 2014 Connecting Washington transportation package.

Needs & benefits

Growth in the Federal Way area has increased traffic at the I-5/SR 161/SR 18 interchange beyond what the original interchange was designed to handle. Each day about 293,000 vehicles use these three highways in this area that is locally known as the "triangle." Frequent backups, particularly during peak travel times, are common.

Several years ago, we started a long-term program to improve mobility and ease congestion in the triangle. Phase 1 built flyover ramps between SR 18 and I-5. The second phase has two parts: 

  • Phase 2A
    • A new exit from southbound I-5 to South 356th Street and SR 161 leading to a new roundabout at the South 356th/SR 161/16th Avenue South intersection. It includes a small roundabout to the east of this intersection.
    • The existing southbound I-5 exit to SR 18 will be rebuilt to provide access to both eastbound and westbound SR 18. Additional lanes on the off-ramp and an auxiliary lane south of 336th Street will be constructed to provide more space for vehicles waiting at the traffic light.
    • The intersection at SR 161 and Milton Road South will be improved by realigning the east leg and rebuilt with dedicated through, left and right turn lanes.
    • This phase will also address six barriers to fish passage along southbound I-5 and SR 161.
  • Phase 2B
    • An auxiliary lane on northbound I-5 north of SR 161 to improve traffic flow.
    • Widen the ramp from northbound I-5 to eastbound SR 18 to two lanes to increase capacity and improve the merge onto SR 18.
    • This phase will address three barriers to fish passage along northbound I-5 and SR 18

During construction of Phase 2A, we will build as much of phase 2B as funding allows. 


  • 2011-2017 - Preliminary design for Phase 2A
  • 2012 - Phase 1 flyover ramps completed
  • 2021-2023 - Design and engineering for Phases 2A and 2B
  • 2023-2027 - Construct Phase 2A


The 2015 Connecting Washington transportation package allocated $85 million for work in the triangle area from 2021-2027. 


Kris Olsen

A map of the I-5/SR 161/SR 18 interchange near Federal Way, Washington.