NMA Email Newsletter: Issue #69

The Diverging Diamond Interchange

The concept was developed in France. (Insert joke here.) Only one such interchange exists in the United States: Springfield, MO activated a diverging diamond interchange (DDI) last June, and says it has seen a 60 percent reduction in accidents at the rebuilt intersection. Because of that kind of safety improvement, and the claim that DDIs are much less expensive to build than traditional interchanges, you may soon be seeing a DDI near you. Several states — Georgia, Kentucky, Maryland, North Carolina, New York, Ohio, Oregon, Tennessee, and Utah — currently have DDI designs in the planning stages. The Ontario Ministry of Transportation is also considering such a design in Toronto.

So what is the DDI concept, and why can it improve traffic safety? Per the diagram below, the key difference is that the opposing lanes of the feeder road to a freeway actually cross each other twice at traffic signals, so that for a short distance, vehicles are driving on the opposite side of the road than is customary.


The primary safety advantage is that no left turns are required to clear opposing traffic. Other advantages are that the traffic signals are two-phase with short cycles thereby reducing delays, the entrance/exit ramps to the freeway require less curvature which reduces the risk of off-road crashes, and, in general, the number of traffic conflict points is reduced.

On the other hand, motorists may be confused initially upon encountering a DDI. Also, the traffic flow on the non-freeway road will be disrupted by having two traffic signals with opposing green and red lights — since they both can’t be green at the same time — a short distance from each other.

Proponents of the DDI design actually combine a variation of the elements of the revenue vs. safety argument that we so often hear: Building a DDI instead of the more traditional design will save substantial cost, and traffic safety will be improved. Let’s hope they are correct.

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