I-5 - SR 161/SR 18 Triangle Improvements

Project news

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Needs & benefits

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

Several years ago we started a long-term program to improve traffic flow and ease congestion in Federal Way around the I-5/SR 18/SR 161 interchange area. Phase 1 built flyover ramps from SR 18 to I-5. Phase 2 has two parts:

  • Phase 2A. A new exit from southbound I-5 to South 356th Street with a new roundabout; rebuild the southbound I-5 exit to SR 18 to expand capacity.
  • Phase 2B. Adding a lane on northbound I-5 to improve traffic flow and a two-lane ramp from northbound I-5 to eastbound SR 18 to increase capacity and ease the merge. This phase is partially funded.


2021-2023 – Design phase 2A and 2B.

2023-2027 – Construct phase 2A and as much as possible of phase 2B.


The 2015 Connecting Washington transportation package allocates $85 million for work in the triangle area between 2021 and 2027. This funding will cover the cost of rebuilding the southbound I-5 interchange with SR 18 and a new southbound I-5 exit and roundabout at South 356th Street, SR 161 and 16th Street South, as well as other improvements in the area.


Kris Olsen

A small map showing a portion of the I-5 / SR-18 / SR 161 Triangle area.