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.
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