Diverging Diamond; a more efficient interchange.

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The DDI is the sort of thing that is easier to visualize than describe (this simulation may help), but here, roughly, is how a DDI built under a highway overpass works: As the eastbound driver approaches the highway interchange (whose lanes run north-south), traffic lanes "criss cross” at a traffic signal. The driver will now find himself on the “left” side of the road, where he can either make an unimpeded left turn onto the highway ramp, or cross over again to the right once he has gone under the highway overpass.

Perhaps counter-intuitively, this complicated approach is actually safer—and more efficient. What makes the DDI work is that it reduces the number of “conflict points” where traffic streams cross each other. There would usually be 26 such points in an intersection like this, but the DDI has only 14 (because, for example, drivers turning onto ramps no longer have to turn across oncoming traffic). But, as Chlewicki explained to me, not having those left-turn movements adds another advantage. In a standard “diamond” interchange, where traffic entering the highway has to turn across traffic, the two sets of traffic signals, because they have to account for the left-turn phase, are difficult to synchronize—which means cars wait in longer queues. But with the DDI, Chlewicki told me, “each signal in the interchange is only two phases, not three. And each of these two phases have some unique characteristics. The left turn from either ramp gets the same green phase as the arterial thru movement that does not conflict with that turn. It’s as if the design doesn’t need a separate ramp phase since it is built into the design.”