WSDOT: Diverging-diamond interchange at I‑90/SR‑18 to open this summer; more short closures planned

Washington State Department of Transportation officials told the Snoqualmie City Council on June 9 that the diverging‑diamond interchange (DDI) at Interstate 90 and State Route 18 will be placed into its DDI configuration this summer, and that the overall interchange‑widening project is expected to finish late in 2025.

The update, delivered by WSDOT public information officer David Rasback and Aecon project manager Kathleen Mahan, listed specific work completed during a recent five‑day closure and outlined additional short closures ahead. "SR 18 was closed for 130 hours. We had a maximum of 120 people working on the site at any one time," Rasback said, and he provided construction totals including 2,700 tons of asphalt, 250 cubic yards of concrete and 10,000 linear feet of striping installed during the closure.

Why it matters: the interchange project widens roughly 2.5 miles of SR‑18 to two lanes each way between I‑90 and Deep Creek, replaces bridges and builds new pedestrian and wildlife passage beneath the roadway. City officials pressed WSDOT on timing and traffic impacts because the route is a primary commuter and emergency‑services corridor for Snoqualmie residents.

Most important facts first: Rasback said the project has opened two of three new bridges — the new Raging River bridge and the new Deep Creek bridge — and that westbound traffic shifted onto new lanes May 28, with eastbound traffic joining them June 1. He said the southern half of the DDI is built and that the next step is completing the northern half. When the next planned five‑day closure under the I‑90 bridges is finished, SR‑18 will operate in the diverging‑diamond configuration.

WSDOT described the detours for the short closures as using eastbound and westbound I‑90 with loops at nearby exits and said it will avoid the Fourth of July holiday weekend and major events such as the PGA tournament when scheduling closures. "We will watch those dates and avoid them," Rasback told the council.

Councilors asked about several community concerns. On pavement striping — an issue raised because some past projects have left faint, confusing “ghost” lines — Aecon's Kathleen Mahan said final paving and the permanent striping will be done together after a required 28‑day curing period for the hot‑mix asphalt, and that the final markings will be applied in their permanent locations to avoid residual ghost striping.

Pedestrian access and fish passage also drew questions. Rasback said a new pedestrian path will be located on the west side of the DDI to allow people to walk under the I‑90 bridges, while the fish pass and open channel will be on the east side. He also said the team opened the fish and wildlife passage beneath the new Deep Creek bridge after removing the old SR‑18 embankment.

Several council members asked whether a future stoplight proposed at the nearby 90 Ninth/parkway area — a separate development proposal — would interfere with the DDI. Rasback said WSDOT's project limits end south of that intersection and the state project will not install or change a signal there: "Our project stops on the south; we don't put a light in there. We don't touch it." He added that any traffic signal tied to private development would be a separate action outside WSDOT's current scope.

What’s next: WSDOT said it expects another five‑day closure in the coming weeks (not over Fourth of July) to finish the north end of the DDI and later a weekend closure, possibly in August, to place the final pavement lift on westbound lanes. Additional ramp and lane closures will occur as needed; the agency said specific dates remain to be set.

Community impact and context: the project includes bridge replacements over Raging River and Deep Creek, the creation of a signalized U‑turn for northbound SR‑18 traffic to reverse toward Auburn, stormwater improvements and new pedestrian facilities. WSDOT noted the contractor coordinated bridge openings and lane shifts to enable fish‑passage and wildlife crossing features beneath the new structures.

Council members praised the project's communications and asked WSDOT to continue notifying residents and emergency services in advance of closures. Rasback said the agency will coordinate with city staff and other local stakeholders when scheduling closures.

A timeline recap: DDI to be opened in summer 2025; SR‑18 widened to two lanes each way between I‑90 and Deep Creek later in 2025; additional short closures expected in the next several weeks; a tentative pavement‑lift closure in August to coincide with annual WSDOT maintenance work.

Details reported at the meeting are based on WSDOT's presentation and council Q&A during the June 9 Snoqualmie City Council meeting.