Diverging Diamond: Novel Highway Design Eliminates Hazardous Left Turns

diverging diamond interchange design

The diverging diamond interchange has been heralded as the new cloverleaf and condemned as pedestrian unfriendly, but it does do at least one thing remarkably well: it eliminates dangerous left-hand turns that tend to cause the worst kinds of accidents.

As confusing as they look, studies so far suggest they reduce crashes by up to a third and fatal crashes by more than half. At first look, these interchanges appear incredibly complex to navigate, but, on the ground, drivers find it easy to follow their directions.

diamond interchange diagram

Drivers never have to turn to get left, which is the most dangerous part of a typical interchange, merging their way into position instead. To accomplish this the roads cross and uncross gradually.

no left turn interchange

While these diamonds date back decades in France, they were only recently imported to the United States. A graduate student named Gilbert Chlewicki thought he had designed the system, only to discover it existed in Europe. There are now dozens of diverging diamonds in over 20 states across the US.

All of their benefits aside, diverging diamonds still have critics. Charles Marohn, an engineer and writer about bad road designs, maintains these interchanges are terrible for bikers and pedestrians. Of course, so are classic clover leafs.

swindon roundabout

Like the infamous Magic Roundabout in England (with five smaller roundabouts circling a larger one), it may just take time for people to get used to the idea, too.