Wider SR 18 and new interchange expected to improve traffic flow

SNOQULAMIE - A pinch point that has kept the state's second diverging diamond interchange from showing its full potential since opening last summer will soon be cleared, allowing State Route 18 traffic to flow.

Contractor crews working for the Washington State Department of Transportation will expand more than 2 miles of SR 18 to two lanes in each direction from Interstate 90 to Deep Creek after a four-day closure of SR 18 over Tiger Mountain scheduled to begin Thursday night, Oct. 16.

If the weekend forecast shows too much rain, the closure could be postponed and SR 18 widening delayed.

What to expect

From 9 p.m. Thursday, Oct. 16, to 5 a.m. Monday, Oct. 20, no eastbound or westbound SR 18 traffic will be allowed between Issaquah-Hobart Road and I-90. Only local access traffic to Southeast 104th Street will be permitted south of I-90 during the closure, and no vehicles will be allowed over Tiger Mountain summit.

A signed detour will use SR 169, I-405, SR 900 and I-90, and people should consider other routes. Freight traffic should use I-405 and I-90 throughout the four-day closure and avoid Issaquah-Hobart Road and Issaquah city streets, which are not suitable for semi-trucks.

During the closure, crews will:

• Pave transitions.
• Stripe lanes in the new configuration with two lanes in each direction.
• Install required barrier and guardrail.
• Shift SR 18, including the intersection with Southeast 104th Street, into its final configuration.

Much of the work needs dry weather, and the closure could be rescheduled if rain is forecast.

Improved traffic flow

The work is part of the I-90/SR 18 Interchange Improvements project, which also opened a diverging diamond interchange in July 2025. The new interchange was designed to work with the additional lanes of SR 18 to improve traffic flow and safety through the corridor.

The new lanes will eliminate a pinch point a few hundred feet south of I-90. Currently, traffic from Snoqualmie Parkway and the eastbound and westbound I-90 off-ramps must funnel down to a single lane to use westbound SR 18 toward Auburn. During busier times, this creates congestion through the interchange that extends up both off-ramps and onto mainline I-90.

WSDOT expects congestion and backups at the I-90/SR 18 interchange will reduce once SR 18 is placed in its final configuration between I-90 and Deep Creek.

The SR 18 widening work will tie into the truck climbing lane just south of Deep Creek, meaning westbound SR 18 will be two lanes to Tiger Mountain summit. This will give westbound traffic approximately 4 miles to calm and merge into a single lane before the highway reduces to one lane.

Project completion

Though SR 18 and the intersection with Southeast 104th Street will be shifted into their final configuration, the project still has several tasks to complete, including:

• Final paving through the I-90/SR 18 diverging diamond interchange, including the ramps.
• Final paving along both directions of SR 18 from I-90 to Deep Creek.
• Permanent lane striping.
• Installing landscaping, curbs and guardrail.

These tasks are scheduled to be completed in 2026 once weather improves.