UPDATE: State Route 18 has reopened under the Interstate 90 bridges after an eight-day closure to switch traffic into the new diverging diamond configuration.
After years of planning and nearly 20 months of construction, the state’s second diverging diamond interchange is almost ready to open at the I-90/SR 18 interchange near Snoqualmie.
To complete the final phase of work, both directions of SR 18 beneath the I-90 overpasses will close from 9 p.m. Thursday, July 17, through 5 a.m. Friday, July 25. Drivers should plan for delays and follow signed detours during this full closure.
This extended closure allows crews to finish critical construction tasks, including drainage and electrical work, curbing, traffic signal setup, and new ramp connections. It also enables a traffic shift into the new diverging diamond pattern, which will improve traffic flow and safety by reducing conflict points in the interchange from 26 to 14. Some of the work planned during the closure depends on dry weather and may be rescheduled if it rains.
Eastbound and westbound I-90 will remain open, but ramps will be limited:
- SR 18 will be fully closed under I-90.
- I-90 off-ramps will allow right turns only.
- Detours will route traffic via nearby interchanges at Preston/Fall City (Exit 22), Highpoint Way (Exit 20), or North Bend Way (Exit 27).
Freight drivers are reminded not to use Issaquah city streets or Issaquah-Hobart Road, which are not designed for semi-trucks. Westbound SR 18 freight traffic should use the Preston/Fall City exit to loop back, or continue to southbound I-405.
Before the Closure
In the days leading up to the eight-day closure, drivers should expect the following ramp closures and traffic impacts:
- SR 18 on-ramp to eastbound I-90 will close from 9 p.m. Tuesday, July 15, to 5 a.m. Wednesday, July 16. Drivers should detour via westbound I-90 and loop around at Preston/Fall City (exit 22) to access eastbound I-90.
- Westbound I-90 off-ramp to SR 18 will close from 9 p.m. Wednesday, July 16, to 5 a.m. Thursday, July 17. Drivers should continue west on I-90, exit at Preston/Fall City (exit 22), and return via eastbound I-90 to SR 18.
Additionally, during the first night of full closure, at 10 p.m. Thursday, July 17, to 5 a.m. Friday, July 18 — crews will restripe all four ramps to support the new diverging diamond configuration. This will involve:
- Rolling slowdowns on eastbound and westbound I-90
- 10-minute traffic holds on:
- Eastbound SR 18
- Southbound Snoqualmie Parkway approaching the interchange
Each ramp will be restriped one at a time during these operations.
After the Closure
When SR 18 reopens on Friday, July 25, traffic will flow through the new diverging diamond pattern, where northbound and southbound vehicles temporarily cross to the left side of the road before returning to the right. This layout reduces wait times and improves safety by eliminating left-turn conflicts and reducing the number of potential collision points by nearly half.
Final paving and permanent striping near the interchange are scheduled to take place later this summer, with specific dates to be announced. Work also continues widening more than two miles of SR 18 south of I-90 and removing six fish barriers, with full project completion expected this fall.
[Information provided by the Washington State Department of Transportation (WSDOT)]
Comments
Great but please get the 4 lanes open to raging river asap. That will dramatically improve the dangerous situation on i90 with backed up cars and reduce congestion. The lanes appear to be near ready. Wsdot?
I wish they would stop calling this an interchange. There are still many stop lights. It will be safer but the time to pass through this area will not be as efficient as a true interchange. The widening of 18 to the south will help, the interchange not so much.
Knock on wood, the whole project will be a NICE / finished improvement!
Just a little work with traffic barriers on the Tiger Mtn climb will be needed a long with the state FIXING pot holes EVERYWHERE and removing their dumb bumpy road signs.
Bill
The exit to Auburn from 90 has a serious issue with the timing of the turn light. Maybe 5 cars can move through the light before it changes to red. However the traffjc coming from the opposite side of the interchange seems to flow without stopping. It appears that the timing of the lights need adjusting. Also it would be useful to have a sign describing “zipper” merging.