Diverging Diamond Interchanges
What is a Diverging Diamond Interchange?
Diverging diamond interchanges are often used at intersections located beneath or above freeway bridges.
At a diverging diamond interchange, traffic briefly crosses over to the left (opposite) side of the roadway, safely guided by traffic signals at each crossover. This allows vehicles to turn left onto freeway on-ramps without stopping and without conflicting with through traffic.
In other words, traffic is briefly routed to the opposite side of the road at an intersection. Because vehicles are now traveling on the left side of the road, they are on the same side of the road as freeway on-ramps and are able to take the freeway on-ramp or other exit without waiting in separate turn lanes as in traditional intersections. Through traffic simply travels through the intersection and switches back to the right side of the road after the intersection, all guided by roadway signs and markings.
Diverging diamond interchanges feature signs, striping, curbs, headlight glare screens and concrete barriers to guide and safely separate opposing traffic as it moves through the interchange.
Driving in a Diverging Diamond Interchange
Driving the Moana Lane (Reno) DDI - a Video Simulation
Driving in a diverging diamond intersection is easy. As you approach the intersection, road signs and striping easily guide you to the opposite (left) side of the roadway and back again to the standard (right) side of the road. If you are exiting the road onto a freeway, you will follow signs to make an easy left-hand turn onto the freeway entrance.
As at any yield, vehicles exiting from the freeway and merging right at the diverging diamond interchange should look closely for and yield to oncoming traffic, as traffic may be approaching from the far left lanes and not the expected nearest lanes. Speed limits may also be slightly reduced in diverging diamond interchanges.
Pedestrians use signalized pedestrian crossings in diverging diamond interchanges and are often directed to a protected pedestrian island in the middle of the interchange. Bicyclists can use the center island or a bike lane if one is provided.
Diverging diamond interchanges can move increased traffic more quickly and safely than traditional intersections- and can often be constructed at a lower cost.
∙ Free, unsignalized left turns (onto freeway on-ramps, etc.) help reduce congestion
∙ Fewer signal phases and shorter wait at traffic lights by eliminating signalized left-hand turn lights.
∙ Diverging diamond interchanges often reduce by half the number of conflict points at which vehicles can potentially collide (by reducing potential crash points between vehicles turning left onto the highway and opposing arterial roadway traffic)
∙ Diverging diamond interchanges can be more cost and time-effective to build, as they often require less land and fewer potentially-costly changes to existing road structures
∙ Heightened pedestrian/bicyclist safety can be provided by designated, enclosed shared use paths built in the middle of the diamond interchange
∙ Makes wrong way entry onto freeway ramps extremely difficult
∙ Snow can continue to be removed/plowed in diverging diamond interchanges
Diverging diamond interchanges have been in use for years in France. The first diverging diamond interchange in the United States was completed in 2009 in Springfield, Missouri, followed by other successful diverging diamond interchanges in states such as Utah. Plans for diverging diamond interchanges are also proceeding in many additional states.
Diverging diamond interchanges were ranked one of Popular Science’s top 25 “Best of What’s New.”
Following installation of a diverging diamond interchange in Springfield, Missouri, a study found:
∙ 46% reduction in crashes
∙ 80% of drivers felt traffic flow improved
∙ 91% of drivers felt it was easy to understand how to drive through the intersection
Federal Highway Administration review of diverging diamond interchanges found an average:
∙ 38% decrease in travel time through the intersection
∙ 80% decrease in number of vehicle stops