A diverging diamond interchange allows two directions of traffic to temporarily cross to the left side of the road. It moves high volumes of traffic through an intersection without increasing the number of lanes and traffic signals. This movement provides easier access to an interstate.
Driving Through a Diverging Diamond Interchange
When driving through a diverging diamond interchange, motorists proceed through a traffic signal at the entrance to the interchange and simply follow their lane to the opposite side of the roadway.
- Motorists accessing the interstate have two options:
- Before they cross to the other side at the traffic signal, drivers may go right to reach the on-ramp.
- After they have crossed to the other side, drivers may simply turn left, without having to stop or wait for any oncoming traffic, to reach the on-ramp to go in the other direction.
- Motorists going straight through the intersection proceed through a second traffic signal and follow their lane back to the right side of the road.
Pavement markings and signals direct motorists to where they need to go.
Pedestrians & Cyclists
Pedestrians use signalized pedestrian crossings and are then directed to a center pedestrian island in the middle of the road. Bicyclists can use a bike lane adjacent to the right lane or a median bike lane if one is provided.
Benefits of Diverging Diamond Interchanges
Safety & Efficiency
A diverging diamond interchange allows free-flowing turns when entering and exiting an interstate, eliminating the left turn against oncoming traffic and limiting the number of traffic signal phases. It is easy to navigate, eliminates last-minute lane changes, and provides better sight distance at turns, resulting in fewer crashes.
The design reduces congestion and better moves high volumes of traffic without the need to increase the number of lanes in an interchange.
In a national study, the design reduced crashes by an average of 37 percent after it was constructed at 26 interchanges across the United States. The design also reduced injury and fatal crashes by an average of 54 percent. (Source: 2019 article published in the Transportation Research Record, the journal for the Transportation Research Board)
Cost-Effectiveness
For improvement projects, a diverging diamond interchange often uses the existing bridge structure and the existing right of way, eliminating the cost of building new structures and purchasing additional right of way.
Because many existing interchange features remain intact, the diverging diamond interchange is often built in less time than it would take to build a new interchange and with significantly less impact to motorists.
History of Use
The first diverging diamond interchange in the U.S. opened in 2009 at an intersection of I-44 in Springfield, Missouri. The Missouri Department of Transportation built the interchange in about six months, maintaining traffic at the location throughout construction.
The N.C. Department of Transportation began using the design in 2014, when three such interchanges opened. Several more have been built, including in Charlotte, the Triad and the Triangle area to better manage traffic flow and growing congestion. NCDOT will continue to develop and build diverging-diamond locations where they solve a transportation problem.