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The Diverging Diamond Intersection

Adding the word ‘cloverleaf’ to the series ‘jughandle, hook, trumpet, and diamond’ should be the ‘dead giveaway’ that intersection design somehow connects them all.  But also prompt wondering whether or not you have driven them all and, also, reflection on how something so seemingly simple as two roads intersecting has generated so many problems and, on the bright side, so many ingenious (and sometimes not so ingenious) solutions.

In this frame of mind, you would likely not be surprised to hear that adoption of yet another intersection design is gradually underway throughout North America, most recently and for the first time in Canada in the city of Calgary.  Although construction is still ongoing, a ribbon-cutting ceremony was held on August 14th where Calgary Mayor Naheed Nenshi said, “Symbolically, a bridge, bringing together two different sides, is a great way for us to continue to build community,” adding that local commuters would no longer be “stolen away from their families” while waiting in traffic.”

Called the ‘diverging diamond intersection’ or DDI, the design links the traditional diamond interchange design with a crossroad design in which the right and left lanes crossover each other at both side points of the diamond.  Crossing through the intersection thus requires all traffic to briefly drive to the left of oncoming traffic before resuming the usual orientation.

No doubt all of this sounds odd and is difficult to conceptualize.  Video simulation is essential for understanding how this design works.  Safe to say, though, that it does and that the kudos it received in its early days—for example, a listing by Popular Science magazine as one of the best innovations in 2009 in the engineering category “Best of What’s New 2009”— are proving well deserved.  

This same website, wholly devoted to explaining and promoting the DDI design provides a detailed list of its benefits, the road engineering ‘trifecta’ of enhanced safety, improved traffic flow, and reduced cost. 

The safety benefits of this novel design are particularly noteworthy and a National Post report of Calgary’s adoption of the design summarized these as follows: “There were initial fears that asking motorists to briefly drive on the “wrong” side of the road would lead to devastating crashes.  However, safety analyses of the new crop of diverging diamonds have revealed the exact opposite. A 2016 study found that, on average, crashes went down by 33 percent.  The rate of crashes that caused injuries fell by an even sharper 40 percent. “Clearly, DDIs offer potential safety benefits,” wrote the report’s authors.”

More than 60 DDI intersections are currently operating in the United States.  Calgary’s DDI is expected to be fully operational this fall. Reports say that Canada’s next DDI is in the works for 2019 in the city of Regina. 

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