Tuesday, July 15, 2025

Diverging from the norm to deliver a diamond

Update: July 2025: Diamonds in the rough

The eight-day closure of State Route 18 July 17-25 will allow the switch to a diverging diamond at the interchange.

By David Rasbach

Eight-day closure of SR 18 will build northern end of diverging diamond

They say diamonds are formed under pressure, and building the diverging diamond interchange at I-90 and SR 18 has been no exception. We know the closures, detours and delays have been tough. But all the time, planning and pressure has gone into shaping something that will last.

From July 17 to July 25, crews will complete the northern half of the diverging diamond interchange at I-90 and SR 18. This work requires another full closure of SR 18 in both directions at the I-90 interchange. It’s longer than the June closure because there’s more work to do, but when the road reopens on Friday morning, July 25, people will begin using the new diverging diamond configuration.

Aerial view of the construction site at the Interstate 90/State Route 18 interchange, with traffic moving through the interchange and trees surrounding the area.
During an eight-day closure of SR 18 under the I-90 bridges, crews will build out the northern end of the new diverging diamond. When we reopen July 25, traffic will be shifted into the new configuration.   

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, July 17, through 5 a.m. Friday, July 25. No traffic will be allowed under the I-90 bridges during this time.

July 17-18 will include rolling slowdowns on I-90 and 10-minute traffic holds on SR 18 and Snoqualmie Parkway as crews stripe ramps and set the stage. As before, emergency responders will be allowed through the work zone. We’re working with freight partners to help them plan ahead and avoid using local roads as detour routes.

What’s happening during the closure?

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

  • Install drainage and electrical crossings
  • Build concrete islands and curbing
  • Set and program new traffic signals
  • Tie in the new I-90 on-ramps
  • Stripe lanes, install signs and switch to the new traffic pattern

This work compresses what would normally take months into a single week, minimizing long-term disruptions and lasting improvements to the corridor.

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
  • No left turns will be allowed from either off-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 July 17-25 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 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 Snoqualmie or 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 southbound I-405. Issaquah city streets and Issaquah-Hobart Road are not suitable for semi-trucks.   

Full detours will use:

  • 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

Opening the diverging diamond marks a major milestone in the I-90/SR 18 interchange project.

Work still remains, as we’ll need to return in the next few weeks to add a top layer of asphalt throughout the diverging diamond, including the ramps. We’ll be back a few weeks after that for final lane striping. We’ll announce the details for that work once it’s finalized.

Though the diverging diamond will be open, it will not fully function as designed to allow traffic to flow until we complete widening SR 18 to two lanes in each direction south of I-90. The project will wrap up that work later this fall, and that’s when we expect you’ll see reduced congestion at this busy interchange. That’s when this diamond in the rough will be ready to shine.

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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