Diverging diamond interchange design won’t meet demands of area traffic | Letter

A diverging diamond intersection may look better, but is no substitute for the traffic-moving ability of ramps and fly-overs.

I am writing to those in the Snoqualmie community and WSDOT design sections responsible for the design and approval of improvements to the I-90/S.R. 18 interchange. The proposed Diverging Diamond Interchange (DDI) concept, I suspect, will eventually be overhwelmed by the traffic demands.

While in the St. Louis, Mo., area this past summer, I traveled through a recently completed DDI on Dorsett Road and I-270. That interchange carries a fraction of the traffic that 90 and 18 carry and during midday, traffic on this interchange were still congested.

I did 30-plus years of service with Washington State in a profession closely linked to transportation and often wondered at the short-sightedness of traffic engineering designs. The original eastbound 520 and 405, and Northbound 405 and 167 interchanges were two good examples. Both clearly illustrate the shortcomings of trying to route crossing traffic on the same plane — it simply doesn’t work.

How Snoqualmie Ridge was developed without corresponding improvements to this interchange leaves one wondering who dropped the ball. It didn’t take clairvoyance to see what lay ahead. Snoqualmie Ridge and North Bend continue to grow, as does the S.R. 18 corridor. Traffic continues to increase and DDIs simply cannot adjust for that increase in volume.

I understand costs as a consideration, but the DDI is a poor substitute for dedicated ramps and flyovers. I would challenge anyone complaining of appearances associated with large concrete interchanges to make an educated decision, a functional highway flyover, or slow-moving traffic slogging its way through a DDI.

John Anderson

Fall City




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