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Sunday, September 9, 2012

Coming soon to an intersection near you

Most of us spend time in traffic. Many of us spend too much time in traffic. When Interstate highways were opening in the 1960s, I remember my Dad's prediction that the problem would be interchanges with surface streets. He was right. Interchanges with surface streets are the site of not only delays but also frequent crashes. They are also very hazardous for pedestrians and cyclists.

The first response of civil engineers was to build flyovers. What followed were concrete monstrosities like the "spaghetti junctions" in Atlanta and many other large metros. They work, but they consume large amounts of land and their cost of construction is enormous. Back to the drawing board!

Innovation produced the Single Point Urban Interchange, or SPUI. North Carolina has a few of them; see this interchange of I-540 and Six Forks Road in Raleigh.
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SPUIs work well, although they can be disconcerting for first-time drivers.

But now there's an even better idea: the Diverging Diamond Interchange, or DDI. Traffic moves through a DDI like crap through a goose, in the purported words of Gen. George Patton. NCDOT is building numerous DDIs. Your first trip through a DDI might make you think you're in the UK or Japan where people drive on the left. Watch this 3-minute video acquaint yourself with a DDI in advance.

Not in North Carolina? You'll see a DDI soon, somewhere.