The diverging diamond interchange: A new kind of intersection eliminates dangerous, time-wasting left turns completely.

How we get from here to there.
Aug. 1 2011 6:53 AM

Don't Turn Left!        

A new kind of intersection eliminates dangerous, time-wasting left turns.

A driver's view of the Dorsett Road diverging diamond interchange looking west. Click image to expand.
A driver's view of the Dorsett Road diverging diamond interchange in Missouri

"Every highway intersection is obsolete," thundered Norman Bel Geddes—the designer and showman perhaps most noted for the Futurama exhibit at the 1939 World's Fair—in his 1940 tract Magic Motorways."The intersection is the chief stumbling block for highway designers and the chief headache for the traffic police," he noted. "Why should the crossroads most heavily traveled today be the ones that are least adapted to the safe flow of the vehicles that use them?"

The question resonates today. In 2007, for example, the Federal Highway Administration reported there were 2.4 million crashes at intersections, representing 40 percent of all crashes, and one-fifth of all fatal crashes. Most intersection crashes fall under the category of "crossing paths," and the most common path-crossing crashes, according to federal statistics, involve left turns.

Left turns are the bane of traffic engineers. Their idea of utopia runs clockwise. (UPS' routing software famously has drivers turn right whenever possible, to save money and time.) The left-turning vehicle presents not only the aforementioned safety hazard, but a coagulation in the smooth flow of traffic. It's either a car stopped in an active traffic lane, waiting to turn; or, even worse, it's cars in a dedicated left-turn lane that, when traffic is heavy enough, requires its own "dedicated signal phase," lengthening the delay for through traffic as well as cross traffic. And when traffic volumes really increase, as in the junction of two suburban arterials, multiple left-turn lanes are required, costing even more in space and money.

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And, increasingly, because of shifting demographics and "lollipop" development patterns, suburban arterials are where the action is: They represent, according to one report, less than 10 percent of the nation's road mileage, but account for 48 percent of its vehicle-miles traveled.

So what, perBel Geddes, can be done? What can you do when you've tinkered all you can with the traffic signals, added as many left-turn lanes as you can, rerouted as much traffic as you can, in areas that have already been built to a sprawling standard? Welcome to the world of the "unconventional intersection," where left turns are engineered out of existence. This is not necessarily a new idea: The "Jersey Jughandle" and "Michigan Left" were early iterations of this concept; rolled out widely in the 1960s, both essentially require drivers to first make a right turn, then either looping back or U-turning their way onto the road onto which they had wanted to turn left.

One brute-force response is to simply build over an intersection. This is what happened at one intersection I know, the corner of Summerlin Road and Gladiolus Drive, in Ft. Myers, Fla. For years, it was a relatively innocuous four-way intersection with a single left-turn lane. My family and I used to stop every year at a small produce stand, Nancy's, on the corner. But as the population of Lee County grew and traffic volumes increased, the intersection became increasingly congested. You were no longer assured of making it through on the first green "arrow"—and perhaps not even the second. And then, one year, as I approached the intersection, I saw a huge concrete loop—reminding me of the monolithic sandworm on the old paperback cover of Frank Herbert's Dunerising out of the ground. There was no longer a left-turn signal—you simply whisked up the ramp, up and over the opposing traffic. There was just one drawback: It was absolutely hideous, like some looming relic of a phantom elevated superhighway. It was totally out of scale, even to the huge arterial roads that surrounded it. Nancy's was a dim memory, the frontage it occupied presumably condemned for right of way.

"Grade separation" is the most extreme way to eliminate traffic conflicts. But it's not only aesthetically unappealing in many environments, it's expensive. There is, however, a cheaper, less disruptive approach, one that promises its own safety and efficiency gains, that has become recently popular in the United States: the diverging diamond interchange. There's just one catch: You briefly have to drive the wrong way. But more on that in a bit.

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The "DDI" is the brainchild of Gilbert Chlewicki, who first theorized what he called the "criss-cross interchange" as an engineering student at the University of Maryland in 2000. (He eventually changed the name for fear of potential confusion with the singer of "Sailing.") Inspired by a similar (and at the time, exceedingly unusual) design in Versailles, France, at the intersection of the Autoroute de Normandie and Boulevard de Jardy *, Chlewicki introduced his concept in a soberly titled paper, "New Interchange and Intersection Designs: The Synchronized Split-Phasing Intersection and the Diverging Diamond Interchange" (PDF) at an engineering conference in 2003.

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.