Science Scene - Diverging Diamond Interchange

America likes imports on its highways: both the cars that travel them, as well as the road designs themselves. 

President Eisenhower created the interstate system in the 1950s after being inspired by Germany's autobahns. Roundabouts, long staples of England and Australia, now exist here in the thousands. And the latest roadway innovation, known as the diverging diamond interchange, comes from France.

These flashily named interchanges eliminate traditional left-hand turns — those that have cars cross lanes for oncoming traffic — by briefly shifting drivers to the left side of the road. Traffic lights allow lanes to safely crisscross at an intersection, giving drivers direct access to the left-hand ramp before putting them back on their normal path. 

The concept is confusing, but engineers see it as a simple way to fight congestion. 

The Federal Highway Administration likes the math. It reports that what it calls "double crossover diamonds" can handle about 650 left turns per hour — twice that of a conventional interchange — while overall delays can be cut by up to 60%.  It can also be cheaper. The compact design allows engineers to increase traffic flow by changing the way they use existing space rather than turning to the costly addition of extra lanes.

 "What makes it the next big thing is it's an inexpensive option," says Don Saiko, project manager for the nation's first diverging diamond, which opened in Springfield, Mo., in 2009. The original plan for expanding that intersection was set to cost $10 million, but the diverging diamond ended up costing only $3.2 million.

Only five diverging diamonds have been completed in the U.S. so far. (Missouri is leading the charge with three, Utah completed one in August and another opened in Tennessee last December.) But Saiko says he has fielded calls from interested engineers and consultants in 35 states, and at least 10 states are building or planning to build them.

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