I-5 - SB CD Lane and 356th St. Intersection Improvements

Project news

  • This project is in the design phase.  It received funding through the 2015 Connecting Washington transportation package. Funding for construction will become available in mid-2023.
  • Do you have questions or comments about the proposed improvements? Email us


A long-term project to improve traffic flow and ease congestion in Federal Way around the I-5/SR 18/SR 161 interchange area is underway. Phase one built flyover ramps from SR 18 to I-5. The next phase calls for a new roundabout and exit on South 356th Street and an southbound I-5 exit to SR 18.

Why is WSDOT proposing to build a roundabout and off-ramp on South 356th Street and a new off-ramp to SR 18?

The area between SR 18, SR 161 and South 356th Street is extremely congested. New ramps and a roundabout will improve driver mobility, access and improve traffic flow.

There are three elements in this stage of proposed improvement:

  • A new roundabout on South 356th Street at the SR 161 and 16th Avenue South intersections.
  • A new southbound I-5 exit to S. 356th Street and SR 161.
  • A new southbound I-5 exit to SR 18. The existing loop ramp to eastbound SR 18 would be eliminated.

New roundabout on South 356th Street at the SR 161 and 16th Avenue South intersections.

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 drivers. Many semitrucks will take up an entire lane between the two intersections, further limiting the mobility other drivers. This congestion then spreads to drivers on other side streets, until the entire area becomes gridlocked. A roundabout will help keep traffic moving.

Southbound I-5 exit to South 356th Street and SR 161

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

New southbound I-5 exit to 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.

This proposal uses the basic footprint of the existing ramp to westbound SR 18 and enlarges it to a five-lane ramp, controlled by a traffic signal. Two lanes will turn right onto westbound SR 18 and three lanes will turn left onto eastbound SR 18.

Previous improvement
When the I-5/SR 18 interchange in Federal Way opened to traffic in the early 1960s its cloverleaf design was considered state-of-the-art in freeway design. But that era's light traffic volumes concealed a major flaw - the weaving that occurs when traffic merges on and off the freeway in the same location. 

The problem wasn't obvious in the 1960s because I-5 and SR 18 each carried less than a quarter of the traffic they do now.

The first phase of this project addressed part of the problem by constructing new flyover ramps from SR 18 to northbound and southbound I-5, eliminating two of the old cloverleaf loop ramps.

The End Result
Two new southbound I-5 off-ramps and a new two lane roundabout will better separate drivers according to their destinations and keep traffic moving through these areas.

Project Benefits

The new roundabout and off-ramps will provide many benefits to drivers:


  • The re-designed off-ramp to SR 18 will accommodate more traffic. Mothballing 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

  • Constructing a roundabout will eliminate the congestion that regularly occurs on South 356th Street at SR 161 and 16th Avenue South. Studies have show that roundabouts can:
    • Reduce vehicle delays by up to 89 percent.
    • Result in a 56 percent reduction in vehicle stops.
    • Increase traffic capacity by up to 50 percent.
  • A new southbound I-5 exit to South 356th Street and SR 161 will ease congestion for drivers. Right now drivers in this area must share the SR 18 ramps, creating significant backups on the ramps, highways and surface streets. A new ramp with direct access legs to South 356th Street and SR 161 will provide better access to drivers depending on their destination.

What is the project timeline?

  • This project is in the design phase.
  • WSDOT held an open house on June 24, 2015 at the Federal Way City Hall to discuss the project with interested members of the community.
  • Construction is tentatively scheduled to begin in mid-2023.

Financial Information

Financial Data for PIN 100502B
Funding Source Amount ($ in thousands)
2003 Gas Tax (Nickel Funding) $0
2005 Gas Tax (TPA) $0
Pre-Existing Funds (PEF) $1,900
CWA $0
Total $1,900


How can I get more information?

Kris Olsen
WSDOT communications