Diverging diamond interchange

A diverging diamond interchange (DDI), also called a double crossover diamond interchange (DCD),^[1][2] is a subset of diamond interchange in which the opposing directions of travel on the non-freeway road cross each other on either side of the interchange so that traffic crossing the freeway on the overpass or underpass is operating on the opposite driving side from that which is customary for the jurisdiction.^[2] The crossovers may employ one-side overpasses or be at-grade and controlled by traffic lights.

The diverging diamond interchange has advantages in both efficiency and safety, and—despite having been in use in France since the 1970s—was cited by Popular Science as one of the best engineering innovations of 2009^[3] and in the U.S. has been promoted as part of the Federal Highway Administration's Every Day Counts initiative.^[4] The flow through a diverging diamond interchange using overpasses at the crossovers is limited only by weaving, and the flow through an implementation using traffic lights is subject to only two clearance intervals (the time during which all lights are red so that the intersection may fully clear) per cycle.^[5][6]

The greatest safety concern of the interchange stems from its relative rarity, as drivers instinctively trying to stay on the customary side of the road could use the crossover intersections to turn against the posted direction of travel. This is a rare occurrence; it is possible only when traffic is so sparse that no cars from the oncoming direction are stopped at the light, and clear signage further reduces the likelihood of such errors.