Cutting-edge intersection slashes congestion, crashes

In a Canadian first, the City of Calgary this week has partially opened a “diverging diamond intersection” that deftly avoids the need for turning left against oncoming traffic.

The $78-million project is expected to dramatically reduce congestion and crashes. At the moment of its inauguration Monday, Calgary Mayor Naheed Nenshi even seemed to get poetic.

“Symbolically, a bridge, bringing together two different sides, is a great way for us to continue to build community,” he said, adding that local commuters would no longer be “stolen away from their families” while waiting in traffic.

And, since it’s Calgary, at the ribbon- cutting workers were al l presented with commemorative belt buckles.

Where Calgarians once had to turn left through a massive four- way intersection, they will now follow a path that weaves them onto the other side of the road.

Anyone t urning left, therefore, doesn’t face any traffic coming from the opposite direction. Drivers making a right, meanwhile, are peeled off before the “weave.”

The only signals at the intersection are two sets of lights at the weave points.

The design is effectively a slimmed- down version of the cloverleaf interchange — the famously convoluted arrangement in which left turns are accomplished by having cars drive on vast circles of roadway.

A cloverleaf requires no lights, but it also takes up village- sized plots of land. Had a cloverleaf been constructed at the site of Calgary’s new diverging diamond, it would have required the demolition of three or four nearby shopping centres.

As highway infrastructure goes, the diverging diamond intersection is surprisingly young. U.S. traffic engineer Gilbert Chlewicki styles himself as the “father of the DDI,” saying he devised the concept in a 2000 term paper — although smallscale examples existed in Europe. The first diverging diamond freeway intersection opened in Springfield, Mo., in 2009, and soon eliminated mile-long queues that had haunted the previous intersection.

Springfield’s diverging diamond was soon dubbed the “wave of the future" and more than 60 U. S. intersections have since followed suit. 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 per cent. The rate of crashes that caused i njuries f ell by an even sharper 40 per cent.

“Clearly, DDIs offer potential safety benefits,” wrote the report’s authors.

Cities are also turning to the design because of cost. In certain cases, a diverging diamond allows a city to speed up traffic with little more than fresh paint and new dividers: No major new overpasses need to be constructed. However, in Calgary, because engineers were working with a standard four- way intersection, they needed to build a new elevated roadway.

Pedestrians and cyclists are funnelled through a protected path running through the centre of the intersection. Previously, anybody crossing the intersection on foot had to slog their way past as many as eight lanes at a time while watching out for left- turning cars. Now, pedestrians only need to worry crossing much smaller one-way lanes of traffic

Already, other Canadian examples of the diverging diamond are on the way. Regina is at work on an intersection expected to open in 2019. Calgary’s diverging diamond is not completely open, however. Portions of the structure will still be under construction until the fall