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‘Diverging diamond’ may ease congestion plaguing I-90 and SR 18 interchange

SNOQUALMIE, Wash. – Drivers on the Eastside could soon see some much needed congestion relief – this time at the Interstate 90 and State Route 18 interchange.

The evening commutes are notorious for backups in the area.  You’ve probably seen them if you drive over the mountain pass.

This project is now years ahead of the original schedule, according to the transportation agency.

WSDOT’s design team will begin next month planning a "diverging diamond." The agency said it would likely be safer and faster for everyone.

The backups begin early along SR 18.

Cars, campers, tractor-trailers -- getting people off and on I-90 from narrow SR 18 has been a pain for drivers as long as they can recall.

When accidents happen near the interchange, traffic can slow to a crawl.

“It’s incredibly congested, slow moving,” said driver Tammy Thieman. “It’s exactly what everybody doesn’t want at the end of a workday.”

The new interchange could look a lot like one planned for an exit on I-5 in Lacey.

It’s called a diverging diamond.

Inside the diamond, traffic in both directions swap sides of the road – which means left turns will be eliminated, keeping drivers moving without having to cross traffic.

Engineers said the diverging diamond means less chances for accidents – and construction costs could be much less expensive than an interchange with flying overhead ramps.

Even though the project is about to begin its design phase, WSDOT said the project could be completed as early as 2023 – the same year the original construction plans were scheduled to begin.