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I-5 - SR 161/SR 18 Triangle Improvements

project status - in design
This map shows the state highways where we built the new flyover ramps.
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This is the new off-ramp to SR 161.  
Ramp opened Sept. 26, 2012

Project Photos

Project graphic
The new interchange
 eliminates weaving and improve traffic flow to and from I-5.


April 2017

We have made major improvements to the area where Interstate 5, SR 161 and SR 18 connect in Federal Way. More information regarding next steps for the this interchange will be available soon.

The reconstruction includes replacing two cloverleaf ramps with flyover ramps, upgraded ramps and improved connections to and from I-5, SR 18 and SR 161.

Due to limited funding, WSDOT built improvements in phases with separate contracts. In the first phase crews:

  • Built a two-lane flyover ramp connecting westbound SR 18 to southbound I-5 and rebuilt the westbound SR 18 to northbound I-5 ramp to accommodate the new flyover ramp.
  • Constructed a new exit ramp connecting the new flyover ramp to SR 161 at S. 359th Street.
  • Built a new eastbound SR 18 to northbound I-5 flyover ramp and rebuilt the eastbound SR 18 to southbound I-5 ramp to accommodate the new eastbound to northbound flyover ramp.

Why is WSDOT making improvements to the I-5 - SR 161 - SR 18 Triangle?
When the Interstate 5/State Route 18 interchange in Federal Way opened to traffic in the early 1960s its cloverleaf design was the state of the art in freeway design. But that era’s light traffic volumes concealed the major flaw of the cloverleaf interchange – the weaving that occurs when traffic merges on and off the freeway. Weaving causes congestion and increases the potential for collisions.

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. Since the 60s King and Pierce counties have grown significantly, pushing the interchange beyond its limits.  The existing loop ramps are substandard and two of the loop ramps are high-accident locations. These improvements will improve traffic flow and safety at this increasingly congested interchange.

The End Result
The project modified the I-5/SR 18 interchange to eliminate weaving vehicle movements. We have selected a series of improvements that will improve existing and future traffic flow, reduce the number and severity of accidents in the vicinity of I-5, SR 161 and SR 18, and avoid or reduce the effects on the environment.

Crews also built a new weigh-in-motion station so truck drivers have the option of bypassing the weigh station on southbound I-5 in Federal Way.

Project Benefits

  • Safety: Improvements to the I-5, SR 161 and SR 18 interchange eliminates weaving (merging) by removing two cloverleaf loop ramps, reducing the number of side-swipe collisions.
  • Congestion relief:  The project has improved traffic flow at this busy interchange.
  • Environmental: To manage stormwater runoff crews built and rebuilt four detention ponds, redesigned a 500-foot stream and turned a 30-foot stream into a 400-foot long, fish-friendly, meandering creek. The project includes 22 wetland sites.

What is the project timeline?

  • April 2005: Began environmental studies.
  • Spring 2005: Selected a preferred alternative.
  • Fall 2005: Began early project design.
  • January/February 2007: Completed the environmental documentation. 
  • August 2010: Crews began construction of the project. 
  • July 16, 2012: Open westbound SR 18 to northbound I-5 flyover ramp.
  • July 23, 2012: Open eastbound SR 18 to northbound I-5 flyover ramp.
  • August 2012: Demolish two old cloverleaf ramps.
  • September 2012: Open ramp from westbound SR 18 to SR 161.

Financial Information

Financial Data for PIN 109907C, 800502K
Funding Source Amount ($ in thousands)
2003 Gas Tax (Nickel Funding) $1,051
2005 Gas Tax (TPA) $81,744
Pre-Existing Funds (PEF) $8,431
CWA $0
Total $91,225

Project signage will reflect the cost of construction engineering, project bid award and sales tax.

This project is funded through the following fund sources.

  • 2005 Gas Tax (Partnership Funding) - $100,000,000
  • 2003 Gas Tax (Nickel Funding) - $3,000,000
  • Existing Funds -  $687,000
  • Other Agency Funds - $8.8 million (federal funds)
  • Total Funding From All Sources - $112.5 million
  • Voters in King, Pierce and Snohomish counties did not approve Proposition 1, which included some funding for this project as part of the 2007 Regional Transportation Investment District Blueprint for Progress. Other sources of money for this project have not been identified.


How can I get more information?
Justin Fujioka

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