I-90 - SR 18 - Interchange Improvements - Diverging diamond interchange

A Diverging Diamond Interchange (DDI) is a proven way to move more people and reduce the potential for collisions by reducing the number of conflict points between vehicles. The first DDI in the United States was built in 2009 in Springfield, Missouri. Since then, over 60 of these types of interchanges are now open to traffic throughout the United States, and another 20 are currently being built.

The first DDI in Washington State will be built at the I-5/ Marvin Road interchange in Lacey, WA. In a traditional diamond interchange, three phases are required to move traffic through the interchange, including a separate phase for left-hand turns. In a DDI, through-traffic and left turns happen at the same time, eliminating the need for a separate left-turn phase. For more information about how DDIs work, visit WSDOT’s YouTube channel to see a DDI in action.