I-5 - SR 161/SR 18 Triangle Improvements - Project details

Phase 2A – Southbound I-5 ramp changes and roundabouts

We are designing upgrades for Phase 2A of the I-5 SR 161/SR 18 Triangle improvements program that will reduce congestion. This work includes three major changes to the area:

A map showing the reconfigured southbound I-5 off-ramp to eastbound and westbound SR 18.

  • A new southbound I-5 exit to eastbound SR 18, which will eliminate the existing loop ramp to eastbound SR 18.
    A larger ramp will accommodate drivers heading in either direction of SR 18. Currently, drivers who want to travel eastbound must take a tight loop ramp and then make a difficult and short merge from the ramp onto eastbound SR 18. The reconfigured ramp will have four lanes, with three lanes for left turns onto eastbound SR 18 and two lanes for westbound right turns.

A map showing the new off-ramp from southbound I-5 to SR 161 at South 359th Street.

  • A new southbound I-5 exit to S. 356th Street and SR 161.
    Right now, I-5 drivers who travel to or from this area generally use the ramps at SR 18 and SR 161. When the South 356th Street/SR 161 intersection becomes clogged, it then starts backing up toward SR 18. A new southbound I-5 off-ramp directly to South 356th Street will provide people with better access to this area and separate them from those who want to use eastbound or westbound SR 18.

A graphic showing the new roundabout at the intersection of SR 161, South 356th Street and 16th Avenue South, and the new roundabout on the ramp from southbound I-5 to south 356th Street.

  • Two new roundabouts; one at South 356th Street at the SR 161 and 16th Avenue South intersections and a second on the southbound I-5 off-ramp to South 356th Street.
    The SR 161 and 16th Avenue South intersections on S. 356th Street are separated by approximately 50 feet. During peak commuting hours, drivers regularly back up into the intersections, blocking other people. This congestion then spreads to nearby side streets, until the entire area becomes gridlocked. A roundabout at that intersection will help keep traffic moving.
    The southbound I-5 off-ramp will include a second roundabout, between I-5 and SR 161, which will slow traffic. If the city of Federal Way decides to build an access road from the new off-ramp to the commercial area that includes Home Depot and Costco, it would tie into this roundabout.

Project Benefits

The re-designed off-ramp to SR 18 will accommodate more traffic. Removing the loop ramp to eastbound SR 18 will eliminate a difficult merge and weaving that occurs, improving safety.

Roundabouts reduce collisions. Studies have found:

  • A 37 percent reduction in overall collisions.
  • A 75 percent reduction in injury collisions.

Congestion relief
Building a new southbound I-5 off-ramp and roundabout at South 356th Street and rebuilding the southbound I-5 off-ramp at SR 18 will reduce congestion at both locations. The roundabout will help move traffic more efficiently through the South 356th/SR 161 intersection and southbound I-5 traffic going to South 356th on southbound SR 161 will no longer need to exit at SR 18.