Tuesday, May 6, 2025

Diverging from the norm to deliver a diamond

Update: 2 p.m. Thursday, May 15, 2025

Canceled: The five-day closure of State Route 18 scheduled for May 15-21 has been cancelled due to forecast rain. It will be rescheduled to a later date.

By David Rasbach 

Five-day closure of SR 18 sets the stage for summer interchange opening

We weren’t planning on a five-day extended weekend closure of State Route 18.
Instead we had a series of normal weekend closures planned over the next few months. But we want to keep the I-90/SR 18 Interchange Improvements project on schedule and moving efficiently, so we’re doing something different.

On Thursday night, May 15, both directions of SR 18 at the I-90 interchange near Snoqualmie will close. It will remain closed around the clock for five full days, reopening Wednesday morning, May 21. If we took the more common approach of overnight or weekend closures, this work would take months. This extended closure allows us to complete critical work in just five days.

We know this will be a major disruption. But it’s a strategic decision that helps avoid many weeks of recurring closures. It also puts us in a strong position to open the new diverging diamond interchange later this summer.

Aerial view of State Route 18 south of Interstate 90 bridges. Cars travel along SR 18 and the on-ramp to eastbound I-90, with orange barrels and construction fencing along the side of lanes. Trees are in the background, with a pond on the left side of State Route 18.
The May 15-21 closure of SR 18 will allow us to complete nearly three months of work on the south end of the new diverging diamond interchange in just five days.

What to expect during the closure

SR 18 will be fully closed in both directions at the I-90 interchange. This closure will be in effect from 9 p.m. Thursday, May 15, through 5 a.m. Wednesday, May 21. No traffic will be allowed under the I-90 bridges during this time.

We scheduled the closure between Mother’s Day (May 11) and Memorial Day weekend (May 26). We want to avoid the busiest holiday travel weekends.

What’s happening during the closure?

Crews will complete a major phase of work on the southern part of the new interchange, including:

  • Installing new drainage and electrical crossings
  • Removing the existing SR 18 pavement
  • Rebuilding and paving the diverging diamond south of the I- 90 bridges
  • Constructing concrete islands and curbs
  • Creating a left-turn lane from westbound SR 18 onto Southeast 104th Street
  • Installing a U-turn with a signal so eastbound SR 18 drivers can head westbound toward Auburn

All this work will be complete when the road reopens on Wednesday morning. The remaining steps will wrap up later this summer. This includes work under the I-90 bridges and on the northern part of the interchange.

This five-day closure allows us to complete in days what would normally take up to three months. It also reduces lane shifts on SR 18, improving safety for crews and people traveling.

Detour and access information

While mainline I-90 will remain open, traffic using on- and off-ramps will be restricted:

  • Eastbound I-90 off-ramp traffic can only turn right (south) onto westbound SR 18
  • Westbound I-90 off-ramp traffic can only turn right (north) onto Snoqualmie Parkway or continue onto westbound I-90
  • No left turns will be allowed from either ramp
  • Eastbound SR 18 traffic must detour via eastbound I-90 and loop around
  • Southbound Snoqualmie Parkway traffic must detour via westbound I-90 and loop around

During the May 15-21 closure of SR 18, you can detour using eastbound and westbound I-90 and loop around at nearby interchanges – exit 20 (High Point Way) or exit 22 (Preston-Fall City) to the west of SR 18 or exit 27 (Southeast North Bend Way) east of SR 18 – to reach your destination.

Maintaining access: Emergency responders, Snoqualmie Valley School District, and Sound Transit buses will be allowed through the SR 18 work zone under the I-90 bridges during the closure.

Freight advisory: Freight haulers heading westbound on I-90 should plan ahead:

  • Use Exit 22 (Preston–Fall City) to loop back to westbound SR 18
  • Or continue west on I-90 to southbound I-405

Please do not use Issaquah city streets or Issaquah-Hobart Road. These streets are not suitable for semi-truck traffic.

Map shows westbound truck detours available during the closure of State Route 18 at Interstate 90 in blue with arrows. One detour continues westbound on Interstate 90 and uses exit 22 (Preston-Fall City) to loop back to westbound State Route 18. The second continues on westbound I-90 to southbound I-405. An orange line shows the route that uses Issaquah city streets and Issaquah-Hobart Road is not an acceptable detour for freight.
Westbound I-90 freight traffic should either use exit 22 (Preston-Fall City) to loop back to SR 18 or continue on westbound I-90 to to southbound I-405. Issaquah city streets and Issaquah-Hobart Road are not suitable for semi-trucks.

Full detours will utilize:

  • Exit 20 (High Point Way) or exit 22 (Preston-Fall City)
  • Exit 27 (Southeast North Bend Way)

We expect delays, especially during peak travel times. Please plan ahead, leave early and consider alternate routes if possible. Real-time traffic updates are available via the WSDOT app and travel map.

Two maps bordered in green showing the closure of State Route 18 in orange and detour routes in blue with arrows. Map on the left has the header “If you’re coming from Auburn and heading to Snoqaulmie …” and shows a detour using eastbound Interstate 90, the Southeast North Bend Way interchange to loop around, westbound Interstate 90 and the off-ramp to State Route 18/Snoqualmie Parkway. Map on the right has the header “If you’re coming from Seattle and heading to Snoqualmie …” and shows the detour using eastbound Interstate 90, the Southeast North Bend Way interchange, westbound Interstate 90 and the off-ramp to State Route 18/Snoqualmie Parkway. WSDOT logo is at the bottom right in white.
People trying to reach Snoqualmie during the closure of SR 18 should use eastbound I-90, loop around to westbound I-90 at Southeast North Bend Way (exit) 27 and exit to SR 18/Snoqualmie Parkway.
Two maps bordered in green showing the closure of State Route 18 in orange and detour routes in blue with arrows. Map on the left has the header “If you’re coming from Snoqualmie Pass and heading to Tacoma …” and shows a detour using westbound Interstate 90, the Preston-Fall City Road Southeast or Southeast High Point Way interchanges to loop around, eastbound Interstate 90 and the off-ramp to westbound State Route 18. Map on the right has the header “If you’re coming from Tacoma and heading to Spokane …” and shows the detour the State Route 18 on-ramp to eastbound SR 18. WSDOT logo is at the bottom right in white.
During the closure of SR 18, people coming from Snoqualmie Pass will have to follow a detour to reach westbound SR 18, while people heading from eastbound SR 18 toward the pass can follow their normal route.
Two maps bordered in green showing the closure of State Route 18 in orange and detour routes in blue with arrows. Map on the left has the header “If you’re coming from Snoqualmie Pass and heading to Tacoma …” and shows a detour using westbound Interstate 90, the Preston-Fall City Road Southeast or Southeast High Point Way interchanges to loop around, eastbound Interstate 90 and the off-ramp to westbound State Route 18. Map on the right has the header “If you’re coming from Tacoma and heading to Spokane …” and shows the detour the State Route 18 on-ramp to eastbound SR 18. WSDOT logo is at the bottom right in white.
During the closure of SR 18, people coming from Snoqualmie and heading to Maple Valley or over Snoqualmie Pass will need to use westbound I-90 and loop around at the Preston or High Point interchanges.
Map with a green border has the header “If you’re coming from Tiger Mountain and heading to Issaquah …” A closure of State Route 18 at the Interstate 90 interchange is shown in orange. The detour is in blue with arrows using eastbound I-90, looping around at Southeast North Bend Way and using westbound I-90. WSDOT logo is at the bottom right in white.
During the closure of SR 18, people coming from south of the I-90/SR 18 interchange who need head west on I-90 will need to use eastbound I-90 and loop around at Southeast North Bend Way.

What’s next

This closure marks a major milestone in the I-90/SR 18 interchange project.

With the southern end complete, we can focus on building under the I-90 bridges and on the northern end of the interchange. This closure helps us take a big step toward improved traffic flow, reduced congestion and enhanced safety. Before you know it we’ll be polishing off a diamond of an interchange.

Artist’s rendition of an aerial view of what the completed Interstate 90/State Route 18 diverging diamond interchange will look like.
Once complete later this summer, the new I-90/SR 18 diverging diamond interchange will improve traffic flow, reduce congestion and increase safety through one of the state’s busiest interchanges.
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