Diverging Diamond Interchange identified as fix for congested I-90/SR 18 interchange area

UPDATE |  May 17, 2017

Back in February 2016 we first told you about a possible unconventional and innovative traffic fix for the congested and often dangerous I-90/SR 18 interchange area.

Now that the Washington State Legislature officially pushed up the project’s $150 million budget – allowing it to begin in 2017 instead of 2023 – it appears that WSDOT will be pursuing a diverging diamond interchange (DDI) as the improvement solution. [Project planning, design and construction is estimated to be complete around 2023.]

As DDI’s are more cost-effective than flyover interchange ramps, the savings will allow for a SR 18 expansion to four lanes on the approach to I-90. 5th District Senator Mark Mullet said the project’s budget will allow WSDOT to widen SR 18 to the Raging River, construct the DDI, and move the truck scales currently located at the interchange.

DDI’s are becoming more common in other parts of the country. WSDOT will begin constructing the state’s first DDI in the Lacey area next year.

You can read more about how a DDI works in our earlier story below.


ORIGINAL STORY | February 3,  2016

Even though Snoqualmie commuters account for only about 17% of the traffic at the 90/18 interchange located just outside of the city limits, in late 2015 the City of Snoqualmie allocated $130,000 of Snoqualmie Ridge Division II mitigation traffic funds and hired Perteet to coordinate and work with WSDOT to come up with interchange improvement proposals – so everything is ready and construction can commence as soon as the state funding is available.

Preferred Interchange Proposal: Diverging Lanes

City of Snoqualmie Parks and Public Works Director Daniel J. Marcinko said the preferred option to improve traffic at the 90/18 interchange would utilize a concept called Diverging Lanes or a Diverging Diamond, which improves traffic flow by eliminating left turns and briefly sending vehicles over to the left side of the road.

The option would require some additional roadway under I-90, but not a lot according to Marcinko, and it would make the most of the existing lanes. The lanes under I-90 would be controlled by a computer system and electronic signage used to alert drivers.

The diverging lanes would then criss-cross each other at a grade – as drivers head up Snoqualmie Parkway to the north and SR 18 to the south – and vehicles would return to the right side of the road.

For drivers this would mean getting used to briefly driving on the opposite side of road, but the pay off would be providing free left turn movements for both east and westbound  I-90 ramps that connect to SR 18 – instead of the current left turn stop lights.  The change  would improve morning traffic when south end drivers are backed up turning left onto I-90 from SR 18, when truck drivers turn left to take freight west on SR 18, and evening commuters heading left toward Snoqualmie Parkway.

90-18 diverging
Aerial photo of the 90/18 interchange with a diverging lane interchange traffic solution. The solution involves providing free left turn movements at the interchange that connects to traffic briefly traveling on the left side of the roadway at the interchange. Traffic then criss-crosses at a grade and returns to the right side of the road after exiting the interchange area.

More Bang for Buck

Marcinko said the City and WSDOT prefer this option as it brings the biggest impact with the least funding – AND leaves additional funding to address a SR 18 lane expansion.

He explained the diverging lane solution is a much cheaper alternative than building an expensive flyover ramp, estimating the cost around $50-$60 million, which would leave the remaining $90 – $100 million in state funding to add lanes to SR 18 between I-90 and the Raging River, something Marcinko says is key to improving traffic at the interchange and in the 90/18 corridor.

And as  a large master planned community is planned for Black Diamond – or about 5,000 new homes and bigger than Snoqualmie Ridge  – getting SR 18 widened to four lanes all the way to I-90 could prove even more important.


Diverging Diamond Interchange example in Florida


Preferred traffic improvement solution at the I-90/SR interchange: Diverging Lanes
Preferred traffic improvement solution at the I-90/SR interchange: Diverging Lanes








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  • interesting how the florida example shows 4 lanes in each direction.
    highway 18 has one lane in each direction with head on traffic.
    this “fix” means cars and trucks just more quickly move into the massive backup — because highway 18 is woefully inadequate for the traffic volume.
    so . . . continued head on driving and stopped cars in the right lane of i90 while others drive 70 mph right next to them. for at least 7 more years.
    it is just unbelievable this is not a higher priority to be fixed.

  • Build a round-about; cheaper, efficient. If you’ve driven in the UK, you know how well they work.

  • Crisscrossing traffic are you insane, as a former engineer for the WSDOT I can see nothing but bad things happening as a result, but what do I know I knew the Lacey V. Morrow bridge was going to sink before that project started and the 99 tunnel would be a disaster.

  • A couple more risk points to this idea are, 1) Semi-trucks exiting I-90 west and going on to SR-18 west, and doing so too quickly resulting in the trailer tipping over. This happens with today’s configuration at least once a month. With the criss-cross that tip over will happen onto cars instead of the guard rail/grass of today’s configuration. 2) People won’t be used to this – how many will continue straight ahead and head-on into oncoming traffic that *they* think is on the wrong side of the road? Even if us locals get used to it, what about tourists? At night. In the rain. Or the snow. Changing the side of the road cars drive on causes problems – ask people in the UK what happens when tourists drive on their roads.

    That said, I’m fully in support of fixing that intersection. It’s bad and getting worse every week.

    1. Roundabouts at both intersections. There is plenty of room for 2 or even 3-lane roundabouts.
      Traffic would crawl, at times; but it would be far faster overall, with the jams starting later and the ‘intersections’/.roundabouts always full of moving cars. Never waiting for the intersection to clear so that a few cars can turn left, etc. Works very well in the UK and in a few problem spots in our state, as well! (Sequim, Issaquah, etc.)

  • I think this is a great idea, and I hope it’s implementation can be hastened. I have driven through one of these in Colorado at the US HWY 36 exit in Louisville. At the time, I was confused and thought it was a temporary detour. However, seeing this plane from above, it looks like it will improve traffic throughput by creating a longer and more efficient merge zone. I also think traffic circles at this exit would be a disaster.

  • Has Marcinko actually sat in our Ridge traffic trying to get onto I-90 while its windy, crappy, rainy weather at 7 am or around that time? He states “average wait times for Snoqualmie drivers to get onto I-90 during the morning commute is about 4-6 minutes. He said the worst case scenario wait is about 6-10 minutes.” My behind it is. I have to plan 20 minutes or more ahead and when the new houses start popping up on the parkway and hundreds more people come out of SE Swenson Drive, it’s going to be even more of a nightmare. I’ve seen traffic back up beyond that street already. Almost to Douglas. I give up most of the time and go I90 E and do a loop around N.Bend Way to go West on I90. Whiskey Tango Foxtrot!!

  • This is a terrifying plan. People, we have sidewalk drivers right now. How in the world will this driving briefly on the wrong side of the road work for new drivers, elderly drivers, those unfamiliar with this format, dark, rain, ice, snow, etc… And these wait times are optimistic. And unrealistic. I have sat on the parkway trying to get through that intersection for 30-50 minutes OFTEN. My co-workers who live down 18 tell me if they aren’t through that intersection by 4:00, their commute increases by at least 30-60 minutes. We need a real, safe solution. Not a terrifying, expensive band aid that common sense tells us will be a disaster.

  • Wow, given the inability for most drivers in the US to navigate a simple two lane roundabout, i.e., the one at the Casino entrance, this should be interesting; and not in a good way.

  • Talk about moving cars, this is a perfect solution to the 90/18 mess. Watch the animation Danna posted of the Florida I-75 example and tell me this is complicated. Here is another example from WSDOT now under construction down in Lacey. https://www.youtube.com/watch?v=5gLxlXamhgY

  • Here is feedback on a real world implementation vs. theory. https://www.youtube.com/watch?v=jzl4POdm0Kk
    As one other poster mentioned; we have enough trouble getting folks to understand how to use a simple roundabout this will be quite interesting to watch. I’m so glad that I’m not dependent on this intersection on a daily basis.

  • Living Snoqualmie