Travelers heading through the Bay Area towards the I-90 corridor should prepare for major disruptions on State Route 18 as the Washington State Department of Transportation (WSDOT) begins a significant road closure to continue constructing a new diverging diamond interchange. The closure is set to start at 9:00 p.m. on Wednesday and will continue through 5:00 a.m. on June 4.
During this period, access to SR 18 beneath the I-90 bridges will be fully closed in both directions. The closure is part of an ongoing infrastructure improvement project that aims to complete the interchange by this summer. WSDOT deliberately scheduled the work after Memorial Day weekend to minimize travel disruption, although weather conditions could delay the timeline.
Traffic on eastbound and westbound I-90 will not be directly affected by the closure, but motorists on SR 18 will be detoured. Drivers heading east on SR 18 will be directed to the eastbound I-90 on-ramp. Southbound travelers from Snoqualmie Parkway will also be rerouted to I-90, using the westbound on-ramp. Detour signage will be in place to assist drivers throughout the closure.
WSDOT issued a special advisory for freight operators: large trucks should avoid Issaquah city streets and the Issaquah-Hobart Road, which are not equipped for heavy vehicle traffic. Instead, trucks are advised to return to westbound SR 18 or take westbound I-90 to southbound I-405 as alternate routes.
Drivers are encouraged to monitor WSDOT’s mobile app and social media channels for real-time traffic updates and detour guidance throughout the construction period.
Once the SR 18 closure concludes on June 4, most of the southern segment of the new interchange will be complete. The remaining work includes construction beneath the I-90 bridges and at the northern end of the interchange. Traffic is expected to shift into the new diverging diamond configuration later this summer.
In addition to the interchange itself, the project includes widening SR 18 south of I-90 and the installation of environmentally friendly bridges to support fish and wildlife passage. WSDOT anticipates completing the expansion by the end of the year.
