The Diverging Diamond Interchange (DDI) seems to be the “next big thing” coming in traffic engineering, and it’s coming to Wyoming.
I’m a confirmed skeptic. The idea of the DDI is to reduce left-turn conflicts and increase traffic through-put by crossing traffic lanes (diverging) across an interchange. If you’re traveling east-bound, for example, you “diverge” from US-style travel-on-the-right to UK-style travel-on-the-left across the interchange then back again after left-turns complete their enter/exit movement.
The DDI looks good on paper. The location where WyDOT is going to install this innovation is a conventional diamond interchange with several large truck stops and serves an urbanizing area of Cheyenne, just north of the Colorado border. All those trucks, combined with tourists and busy commuters, can be a mess.
There’s a similar interchange off I-90 on the east side of Rapid City, South Dakota, that I’ve regularly stopped at the last several years. Big truck stop, fast food, short-cut into town, I’ve sat and waited out congestion there so I was glad to see new construction in process. SD DOT built what they call a “Single Point Urban Interchange”, similar to a diverging diamond, but with a big, wide bridge and big, long off-set left turn lanes, that are utterly confounding, confusing, and I’m sure very, very efficient in moving traffic.
Modern round-abouts in the US have been criticized for similar reasons. They’re new! They’re different! People don’t understand them! I haven’t bought those arguments against roundabouts—people get used to them. I don’t want to summarily dismiss the DDI in the same way. What we have isn’t working so well so we have to look at new ideas.
However, some other really smart people have criticized the Diverging Diamond as just more lipstick on the pork barrel politics of our modern transportation system. It may be better bacon, but it’s still a pig.
Wyoming DOT is hosting an open house Thursday 17 January, 5-7pm, at the Romero Community Center, 1317 Parsley Blvd, Cheyenne. If you’re in the area, you might want to check it out.