I-90/SR 18 Interchange Design

This is a four-lane Diverging Diamond Interchange (DDI) with a 12-foot shared-use path on the west side of SR 18.

Click and drag the two-way arrow to see how the new solution looks.

Existing condition

Diverging Diamond Interchange


  • Two northbound and two southbound lanes through the interchange.
  • A dedicated northbound right turn onto eastbound I-90.
  • Two left-turn lanes for eastbound and westbound I-90 off-ramps.
  • Two right-turn lanes from eastbound I-90 to southbound SR 18.
  • Westbound on-ramp can be metered during peak hour operations to allow for better traffic flow.
  • Traffic signals allow for safe movement through the interchange.
  • Pedestrians and bicyclists benefit from dedicated shared-use path signals.
  • No need to rebuild the I-90 westbound overpass.


  • Traffic models show up to a 15% reduction in total crashes and a 36% reduction in fatal crashes compared to not improving the interchange.
  • Traffic models show a four-lane DDI will prevent significant back-ups through the interchange in morning and afternoon peak traffic periods and both directions until at least 2045.

What is a Diverging Diamond Interchange?

A Diverging Diamond Interchange (DDI) moves more people and reduces the potential for collisions by reducing the number of conflict points between through traffic and turning traffic. The first DDI in the United States was built in 2009 in Springfield, Missouri. Since then, over 60 of these types of interchanges have opened throughout the United States, and another 20 are being built. The first DDI in Washington State is being built at the I-5/Marvin Road interchange in Lacey, WA.

Above you will find videos that explain how the DDI at Marvin Road will work. An important difference between the Marvin Road DDI and the I-90/SR 18 interchange DDI is that the Marvin Road DDI is being built over the interstate. The DDI at I-90/SR 18 will be built under the interstate, making it the first of its kind in Washington State.