After a few millennia of making roads, you'd think humans would've figured it all out by now. And yet, even the past decade has seen thoroughfare innovations come to the US. Since 2009, a new kind of interchange geometry has swept the nation, popping up 62 times in 22 states, with more under construction. Dubbed "diverging diamonds," these fancy-looking road arrangements are a clever tool for neutralizing the most dangerous element of any high-speed intersection.
The strength of the diamond intersection is its zero tolerance policy on left turns into traffic, says Carlos Sun, a civil and environmental engineer with the University of Missouri. Left turns aren't just dangerous---forcing drivers to risk particularly violent head-on and 90-degree crashes---they're inefficient. In busy intersections, through traffic loses valuable green light time to those safety-minded left turn arrows. "You're wasting green light by accommodating the left turn," says Sun.
So the diamond intersection, ideal for spots where surface roads intersect with highways, and where cars move between the two, gets rid of the risky process of turning left across oncoming traffic. There's a lot going on visually, but the system's pretty simple. Say you're coming from the east side of the road: First, you wait at a red light as drivers from the west smoothly criss-cross from one side to another. Then their light turns red, and it's your turn turn to move through the elongated ichthys. More cars merge and exit the streams through wide turns. Meanwhile, a highway travels above the whole dance, allowing even more vehicles to move through a relatively constrained area. Traveling between freeways has never been so hypnotic:
The Columbus of diverging diamond interchanges is a chap named Gilbert Chlewicki. As a grad student back in 2000, Chlewicki wrote a term paper on the new traffic arrangement. Then he took a trip to France and was startled to find his tour bus smoothly rolling through a familiarly diamond-shaped intersection outside of Versailles. In fact, France has had a few of the things since the 1970s, but it was Chlewicki's "rediscovery" that triggered their newfound popularity in the US. So far, they've been especially hot in states like Utah and Missouri, where traveling wide distances is a way of life.
Last year, Sun and transportation specialist colleagues from across the country teamed up to perform the first in-depth safety analyses comparing the diverging diamond interchanges to the common classic diamond setups, which force exiting cars to make turns across traffic. The geometric oddities perform about exactly as well as the researchers hoped. Of the seven intersections examined, five saw serious safety improvements. Overall, the engineers estimated diverging diamonds should reduce crashes by 33 percent. What crashes remained were much less likely to kill. Fatal crashes on terminal ramps fell by over 60 percent. Property damage fell by half. And while these things look like they should leave drivers more confused than a cow on astroturf, researchers find that drivers very rarely go the wrong way on DDIs. When drivers do make mistakes---often at nighttime---the researchers' observations show the what crashes do occur are not often fatal.
In a country where even the well-vetted roundabout can be considered an out-there traffic management idea, diverging diamonds are catching on quick. Sun and his colleagues say more research is needed before calling them the safety king of intersections, but preliminary results are looking good. If only the US devoted such dazzling feats of geometric mastery to, say, bike lanes.
