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."
The DDI didn't just go from Chlewecki's presentation into asphalt. The concept was extensively tested on driving simulators by the FHWA's Human Centered Systems team, who worried that "human errors due to unfamiliarity" might lead to more crashes (one suggested design element was a special "Keep Left" arrow, rather than the standard R4-8 "Keep Left" sign, which is more commonly used to signify "Keep Right").
But drivers by and large got it, and in 2009, the nation's first DDI went up in Springfield, Missouri. There are now any number of DDIs built or in the works, in states from Tennessee to Minnesota to Utah. Chlwecki admits they aren't always appropriate; they don't work well when traffic is equally heavy from opposing directions. Some truckers have pointed out that the design doesn't allow one to exit the highway and then quickly re-enter by proceeding straight across the intersecting street to the next on-ramp (if one has to check calls, or has gotten off at the wrong exit). And aesthetically, like many large-scale engineering works, they are perhaps best appreciated from above. While the intersections are avowedly built with access for pedestrians and cyclists in mind, as this rather involved walk-through video of a DDI reveals, it doesn't really feel like a human-scaled environment.
In a terrain that has been largely ruled by designs like the cloverleaf, the conventional diamond interchange, or, most recently, the early 1970s vintage single-point urban interchange—which was the only one to tackle the left-turn problem (though not as effectively, in cost or reduced conflict points, as the DDI)—there is presumably no end to the amount of intersection innovation still to come. One new solution, the "Continuous Flow Intersection," is almost as common as the DDI itself. Chlewecki notes nearly three dozen alternatives to the traditional four-way junction, ranging from the "echelon" to the "windmill interchange" to the modern roundabout. (Like the DDI, the roundabout is often initially the subject of some vitriol from drivers; it could theoretically be used in place of a DDI, but doesn't seem to perform as well for large interchanges).
But one wonders whether these new alternatives will eventually run up against the same problems faced by intersections with traditional left-turn lanes. In one Missouri study, a model that forecast traffic volumes at a DDI interchange in the year 2035 found that "two of the DDI models had slightly lower corridor travel times than those of the typical diamond interchange." But here too is the limitation in the "predict and provide" mindset of traffic modeling: It presumes that traffic volumes will be much greater in 2035 because we will keep building houses with three-car garages, towns without sidewalks, eight-lane suburban arterials, and things like CFIs and DDIs to try to keep pace with traffic, rather than enabling and effecting more efficient pairings of land-use and transportation. While a fascinating engineering solution in its own right, the DDI is haunted by a lingering question: Can you ever truly design your way out of congestion?
Correction s, Aug. 1, 2011: This article originally misidentified the Boulevard de Jardy. ( Return to corrected sentence.)A photo caption on this article originally placed the Dorsett Road interchange in Springfield, Mo., and stated that it was the nation's first diverging diamond interchange. The Dorsett Road interchange is near St. Louis, and it is not the first.