Diamonds are forever: New I-90/SR 18 interchange to open after eight-day closure of SR 18

SNOQUALMIE – After several years of planning and design, and nearly 20 months of construction, it’s almost time for the state’s newest diamond to shine.

The Washington State Department of Transportation will open the second diverging diamond interchange in the state after an eight-day, round-the-clock closure of both directions of State Route 18 under the Interstate 90 overpasses scheduled to begin Thursday night, July 17. 

From 9 p.m. Thursday, July 17, through 5 a.m. Friday, July 25, traffic will not be allowed

along either direction of SR 18 beneath the I-90 bridges. People traveling through the I-90/SR 18 interchange during the closure should expect delays, especially during peak travel times, and follow signed detours.

Longer closure with more work

Crews built the southern end of the new diverging diamond, which is part of the I-90/SR 18 Interchange Improvements project, during a five-day closure in early June. There’s even more work to do during this second extended closure, which will last eight days.

Much like the work finished in early June, the extended closure condenses the time needed to complete work and eliminates the need for multiple weekend and nighttime closures. It also reduces the number of traffic shifts, which improves safety for workers and people driving through the work zone. 

During the closure, contractor crews working for WSDOT will:

• Place new drainage and electrical crossings
• Build concrete islands and curbing
• Position and set timing for new traffic signals
• Complete the new I-90 on-ramps
• Install temporary lane striping and new signs to guide people through the interchange
• Switch SR 18 traffic into the diverging diamond traffic pattern

Some of the work during the closure needs dry weather and may need to be rescheduled if it rains.

What to expect 

Eastbound and westbound I-90 traffic will not be affected during most of the closure, but people traveling through the area should be aware that:

• Vehicles will not be permitted on either direction of SR 18 beneath the I-90 bridges.
• Traffic on the eastbound and westbound I-90 off-ramps will only be able to turn right.
• Eastbound SR 18 traffic must use the eastbound I-90 on-ramp and follow a detour.
• Southbound traffic on Snoqualmie Parkway must use the westbound I-90 on-ramp and follow a detour.

Detour options

People can detour around the closure using eastbound and westbound I-90 and loop around at nearby interchanges – Preston/Fall City (exit 22) or Highpoint Way (exit 20) to the west and Southeast North Bend Way (exit 27) to the east. 

Westbound I-90 freight traffic should use the Preston-Fall City exit to loop back and take westbound SR 18 or continue on westbound I-90 to southbound I-405. Freight traffic should not use Issaquah city streets or Issaquah-Hobart Road, which are not suitable for semi-trucks. Similarly, eastbound SR 18 freight traffic cannot use Issaquah city streets to access I-90.

Before the closure

To help prepare for the switch to the diverging diamond pattern, crews need to install a permanent barrier along two of the four ramps that will remain open during the eight-day closure. To allow for this, travelers should expect ramp closures on the nights leading up to the extended closure:

• SR 18 on-ramp to eastbound I-90: Closes from 9 p.m. Tuesday, July 15, until 5 a.m. Wednesday, July 16. People should detour using westbound I-90 and loop around to eastbound I-90 at Preston/Fall City (exit 22).
• Westbound I-90 off-ramp to SR 18: Closes from 9 p.m. Wednesday, July 16, to 5 a.m. Thursday, July 17. People should continue on westbound I-90, loop around at Preston/Fall City (exit 22) and use the eastbound I-90 off-ramp to SR 18.

All four ramps also need to be restriped for the diverging diamond traffic pattern during the first night of the eight-day closure. People traveling through the area between 10 p.m. Thursday, July 17, and 5 a.m. Friday, July 18 should expect a series of rolling slowdowns along eastbound and westbound I-90 and 10-minute traffic holds on eastbound SR 18 and southbound Snoqualmie Parkway approaching the interchange. Crews will update the striping on each ramp, one at a time. 

After the closure

When SR 18 reopens Friday, July 25, SR 18 traffic will use the diverging diamond traffic pattern, where northbound and southbound traffic cross to the left side of the highway to go under the I-90 bridges before crossing back over. This eliminates vehicles turning left in front of on-coming traffic, allowing for fewer traffic signal phases and improving traffic flow. It also reduces the number of places where vehicles could possibly collide in the interchange from 26 to 14, improving safety.

WSDOT opened the state’s first diverging diamond interchange in 2020 at the I-5/SR 510 (Marvin Way) interchange in Lacey. 

Crews will still need to add a final layer of asphalt to the interchange near Snoqualmie and permanent lane striping later this summer. WSDOT will announce details about that work when plans are finalized. 

In addition to building the new interchange, the project is widening more than 2 miles of SR 18 south of I-90 to two lanes in each direction and removing six barriers to fish passage. SR 18 widening is expected to finish later this fall.