Bay Area commuters have a reason to rejoice as Washington State has opened its second diverging diamond interchange yesterday. The intersection where Interstate 90 meets State Route 18 near Snoqualmie now features a fresh traffic pattern aimed to significantly improve both flow and safety—offering a promising solution to notorious traffic snarls.
The Washington State Department of Transportation (WSDOT) was to officially launch the new interchange yesterday morning, following an intense eight-day closure required to construct the northern end of the configuration. The innovative design enables drivers on SR 18 to temporarily cross to the left side of the highway while passing beneath I-90 bridges—a maneuver designed to eliminate the need to turn left across oncoming traffic, thereby reducing collision points and smoothing out traffic transitions.
But this isn't the first rodeo for diverging diamonds in the state. Washington State Department of Transportation opened its inaugural interchange of this type back in 2020 at the I-5/SR 510 (Marvin Way) in Lacey, following in the footsteps of the first-ever such interchange in Springfield, Mo., in 2009. With over 200 such interchanges now spread across the nation, it's a tried and tested approach to traffic management that holds much promise.
Motorists traveling through the interchange can expect to navigate a system of clearly marked signs and lane indicators, including freshly painted lines and synchronized traffic signals to ensure they swiftly reach their destinations without hassle. Among the most notable features are the salmon-colored concrete islands, strategically placed to guide vehicles through the crossover points.
The completion of the interchange marks a crucial milestone in the extensive I-90/SR 18 Interchange Improvements project. Yet, the work is not entirely wrapped up. A top asphalt layer across the interchange remains to be added, with permanent lane markings to follow suit. Washington State Department of Transportation plans to reveal the dates for this final phase of construction in due course.
Improving traffic flow is just one side of the coin; the new interchange opens the gates to a more extensive SR 18 widening project. When completed later this fall, the route will expand to two lanes in each direction for over two miles, facilitating a streamlined journey for both local travelers and commercial freight between southern and eastern King County. Added environmental benefits include the removal of fish barriers and the establishment of wildlife corridors for various species. The entire project, with a robust budget of $188 million—mostly drawn from gas tax funds—is on track to reach its completion by late 2025.
