An Ingenious Highway Interchange Is Spreading Across the U.S.

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When construction ended and the interchange opened in June 2009, there were groans and grumbles galore. Engineers had implemented a radical new design – the first of its kind in the United States – where Interstate 44 and Route 13 meet in Springfield, Missouri, and many drivers weren't enthused with being guinea pigs. Motorists on Route 13 were now forced to stop at a traffic traffic light, then follow the snaking road as it gently curved left and straightened out. For a fleeting few seconds as they crossed the bridge over Interstate 44, motorists drove on the left side of the road – like they were in England!* Some smoothly turned left to travel onto I-44, while those continuing on Route 13 crossed under another traffic light as they veered right and exited the interchange back on the right side of the road, their experience with the diverging diamond interchange (DDI) now concluded.

More than ten years later, the groans and grumbles are mostly gone as what was novel has now become mundane. There's also simply much less to complain about. Traffic flow has significantly improved and crashes are way down. Why? Because with a DDI there are no left turns that require drivers to cross an oncoming lane of traffic.

The Missouri Department of Transportation has now constructed five more DDI interchanges around Springfield, partly because they were 75% cheaper to build than traditional diamond exchanges. Subsequent analyses showed that they reduced fatal crashes by at least 55% and total crashes by roughly 40%.

The U.S. Department of Transportation's Federal Highway Administration also touts DDIs. They reference a study which compared DDI designs to conventional diamond interchange designs under a range of high and low traffic volume combinations. For higher traffic volumes, the DDI designs reduced delays by 15-60 percent and increased throughput by 10-30 percent.

These outstanding results explain why a DDI interchange is likely coming to an intersection near you: 109 are currently in operation in the United States, with 33 presently under construction, and 153 more either planned or proposed. They are most frequently utilized where a minor road or highway meets an interstate.

One of the chief architects of the diverging diamond revolution now underway is Gilbert Chlewicki, who formulated the design back when he was a first year graduate student in transportation engineering at the University of Maryland. He had been sketching road designs for fun since elementary school, like an artist drawing doodles. While Chlewicki thought his idea looked good on paper, he wasn't sure whether or not it would work in practice until he actually traveled through one when he was on a bus tour in France. It turned out that the country already had two of them in operation since the 1970s. Though bummed to realize that he didn't originally come up with the idea, Chlewicki grew determined to see them implemented United States. He continued to write papers about DDI and speak at conferences after he graduated and entered the workforce. DDI garnered a fair amount of interest, but ultimately, no transportation official wanted to take the risk on an untested interchange design.

Don Saiko, a project manager with the Missouri Department of Transportation, was the first to dive in. He and Chlewicki were both present for the ribbon-cutting ceremony in Springfield and now share in the satisfaction of DDI's much broader success.**

Local news stories in the U.S. now herald the construction or completion of "trippy" new DDI interchanges on a weekly basis.

"It was kind of odd at first, because you're switching sides, but I'm used to it," Phillip Barnett, a sixty-year resident of McCracken County, Kentucky told WPSD-6 of a new diverging diamond that was recently built in Paducah. "I love it."

"I can remember trying to get through the traffic prior to it being built, and it seems like this is a whole lot speedier. It's faster."

*Corrected 12/10. The passage originally insinuated that Europeans drive on the left side of the road. In fact, most Europeans do not.

**Section updated 12/11. It originally stated that Saiko and Chlewicki worked closely together during construction of the Springfield DDI. That is incorrect.

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