What is a Diverging Diamond Interchange?
July 22, 2019 •Tim Kaddatz
Roughly 10 years ago, we started to see traffic circles (or "round-abouts") pop up across the US. What is a traffic circle? Well, if you don't know, a traffic circle is exactly what is sounds like; it is an area of traffic designed in a circular pattern. It allows for the flow of traffic to be continuous for those within the circle and for traffic to safely enter the traffic circle when there is no traffic approaching from the left. Traffic circles reduce the likelihood of side impact collisions found at the traditional 90-degree intersection. They also eliminate the need for electronic traffic control as the traffic circle is its own source of traffic control. The Department of Transportation (DOT) claims a 75% reduction in injury collisions and a 90% reduction in fatal collisions as a result of traffic circle creation. Simply put, traffic circles are a safer alternative to the traditional intersection and they also allow for a smoother flow of traffic.
The purpose of this blog, however, is not to give you a refresher on traffic circles. The purpose is to introduce you to something new. That something new is called the "Diverging diamond interchange."
A diverging diamond interchange is a type of diamond interchange in which the two directions of traffic on the "non-freeway" road cross to the opposite side on both sides of the bridge at the freeway. It is unusual in that it requires traffic on the freeway overpass (or underpass) to briefly drive on the opposite side of the road from what is customary. The diverging diamond interchange allows for two-phase operation at all signalized intersections within the interchange. This is a significant improvement in safety, since left turns need not clear oncoming traffic.
Here is an overhead view of what these look like in regards to traffic flow:
And here are some simple tips to remember which should allow for safe navigation at these types of intersections:
- Follow the signs, signals, and road markings to cross through the intersection at the first set of traffic lights. Traffic will look like it does on a one-way street.
- All left turns onto the freeway are free flowing.
- Vehicles going straight (no plan to turn onto the freeway) simply proceed through a second set of traffic lights.
- Pedestrians should travel on designated walkways and cross only in crosswalks.
- Bicyclists may choose to use the bike lane or pedestrian walkways and crosswalks.
The diverging diamond interchange has been used in France since the 1970's, however it is relatively new to the U.S. Much like the traffic circle, you may find these coming to a town near you in the near future. If you travel, you may already find these during your travels. Be alert, be prepared, and stay safe!