Laura Bliss is CityLab’s west coast bureau chief, covering transportation and technology. She also authors MapLab, a biweekly newsletter about maps (subscribe here). Her work has appeared in the New York Times, The Atlantic, Los Angeles magazine, and beyond.
Drivers may be baffled by these newfangled intersections, but they’re safer than traditional four-way stops.
End-of-summer road trippers: Get ready to merge like never before. Diverging diamond interchanges are coming to a highway near you.
The brainchild of an engineering graduate student in the early 2000s, diverging diamond interchanges, or DDIs, work rather ingeniously. Rather make drivers turn at a right angle to merge onto a highway, the two directions of traffic diverge and ribbon over one another. One lane peels off to funnel cars onto the highway, and they never have to turn against traffic.
In the past two weeks alone, Florida opened all 12 lanes of the nation's largest DDI near Sarastota. Near Pittsburgh, a brand new DNA-shaped interchange is confusing some Pennsylvania drivers and delighting others. Southeast Calgary opened another one this week, Saskatchewan awaits its own, there’s news that Arizona and Virginia are considering proposals to build out more. Here’s a full list of nearly 90 U.S. locations where they’re planned or operational.
DDIs are proliferating because they’re safer than a traditional four-way intersection. Where two, two-lane roadways intersect, drivers have 32 separate opportunities to collide into each other. In a DDI, there are only 14. The DDI in Springfield, Missouri—the first in the U.S.—showed a 60 percent reduction in crashes since it was installed in 2009, compared to the old design. They can also be more cost-efficient than traditional diamond interchanges.
Urbanists might grump that there’s no such thing as a “good” highway onramp, but some DDIs come with walking and biking paths alongside the twisting lanes (separated, of course), with engineers applauding themselves for the freer flow of feet and bikes created. (‘Cause if cars don’t need to stop for traffic, neither do people.) And with traffic fatalities rising at alarming rates and funding to rebuild decaying roads getting tighter, the DDI seems like a positive trend for both drivers and DOTs. "Human beings, I know, hate change,” Pennsylvania Governor Tom Wolf said on the occasion of his state’s first. “But this really works.”