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Wednesday, November 12, 2008
That Diverging Diamond

This post about a clever piece of engineering coming to American Fork even has embedded YouTube video, which is a first here at the blog. (I love technology.)

"Diverging diamond" sounds like it might be a solution to the all-important question, Who gets the wedding ring in the divorce settlement? It's actually a happier thing than that, in this blogger's dubiously humble opinion. You may have heard of it already in the last week or so; I first heard of it a week or two before that, but I was too busy holding forth about the election to mention it then.

MFCC brought an 11x17 artist's conception home from a recent City Council meeting. I studied it for a few minutes and pronounced it a very clever piece of engineering. I liked it so much that I took it to work with me and showed it to a couple of engineers I work with; they liked it, too.

"It" is a design for a freeway interchange that is planned for I-15 at American Fork's Main Street. The current, more conventional interchange there is no longer adequate to current traffic and would be even less so with the upcoming addition of a major new road, the so-called Pioneer Crossing, meeting I-15 there from the west. The most noteworthy thing about the diverging diamond is that traffic exiting the freeway (either north- or southbound) can go either direction on the intersecting roadway without turning across lanes going the other direction. Traffic can also enter either direction of the freeway with a similarly "unopposed" left turn. It's not magic, just clever engineering.

Utahans are now familiar with the SPUI, the Single Point Urban Interchange, such as the one at University Parkway and I-15 in Orem. It is said to be 30 percent more efficient than conventional interchanges, in terms of moving traffic. The diverging diamond is 30 percent more efficient than that, and it has the added virtue of requiring less land and less concrete. It is currently in use in a couple of locations in France.

American Fork's diverging diamond will be either the first or second completed in the United States; the other is in Missouri and is slated for completion the same month.

Recent media coverage includes these articles in the Salt Lake Tribune (also here), in the Cleveland Plain Dealer (where I found the YouTube video embedded below), and at KSL.com.

The media make it sound more confusing than it will be, with talk of driving on the left side of the road. It will be more a matter of simply driving the right way on a one-way road. Opposing traffic is on a separate bridge. So it won't be confusing from the driver's seat, as it may appear from the air.

(The American Fork design puts the freeway under the intersecting road. In the video, the freeway is over the intersecting road. Either way, the concept is essentially the same.)

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