Have you ever encountered a highway interchange that allowed you drive on the left side of the roadway? While you may have briefly felt like you were in Europe, you were moving through what’s known as a Diverging Diamond Interchange (DDI). A DDI allows for the two directions of traffic on a freeway overpass (or underpass) to briefly drive on the opposite side of the road from what is customary. DDIs work best when one of the left turning movements is high and/or if thru movements are unbalanced during peak hours. So what’s the story behind this unique interchange design?
History of the Diverging Diamond Interchange
The concept of the DDI emerged from searching for creative design options that could provide short term solutions at a much lower cost than typical interchange or bridge replacement projects. Gilbert Chlewicki, a transportation graduate student at the University of Maryland, published a paper in 2000 about diverging diamond interchanges, and he is largely credited with “modernizing” the current DDI design used in the United States. Prior to 2009 the only known diverging diamond interchanges were in France, and they were built in the 1970s.
The first DDI in the United States was built in Springfield, MO in 2009. While the cost to widen and replace the bridge at this location was estimated at over $10 million, the cost to construct this DDI was only $3.2 million. Because of this successful design many more DOT agencies across the county began to study and build DDI locations, and it is being increasingly recognized as a cost-effective transportation design solution. There are approximately 95 DDIs operational in the world, with roughly 90-95% of those being in the United States.
Those familiar with the metro Atlanta area may have driven through one of Georgia’s DDIs at the recently opened interchange on Windy Hill Road at I-75. There are four DDIs in metro Atlanta, with a fifth being located off I-95 in Chatham County. Georgia will soon add a sixth DDI with the completion of construction for the DDI at I-75 at Wade Green Road in Acworth.
Advantages of Diverging Diamond Interchange Despite a movement that requires drivers to drive on the opposite side of the road, studies have shown this design of roadway layout and approach angles, use of medians and islands, and additional signage have actually helped reduce driver confusion. The DDI design has also demonstrated other safety benefits such as reducing intersection conflict points, minimizing the ability to make Wrong Way maneuvers onto ramps, and reducing speeds through the intersection.
The most significant DDI design feature involves the reassignment of the ramp terminal intersection mainline left turn movement to the outside lanes. This allows for mainline left turn movements to operate free flow onto the interstate or expressway. This left turn maneuver is removed from the intersection signal operation and therefore allows the interchange to operate as a two-phase operation. This type of signal operation can improve efficiency, decrease cycle times for the intersection, and reduce lost time associated with traffic signal change intervals (vehicle startup times and clearance interval times associated with red and yellow times). Allowing this left turn movement to be free-flow and to be separated from the intersection movements also reduces the number of conflict points associated with vehicular turning movements, therefore improving safety at the intersection.
Additionally, there is a cost benefit shown to be associated with the DDI design and operation. Traditional diamond interchanges have incorporated left turn phases and storage lanes into the interchange intersection design. The storage for the vehicles turning left onto the interstate has typically been handled with adjacent opposite direction lanes on or under the interchange bridge. With the free-flow left turn lanes reassigned to the outside lanes, the need for storage lanes has been eliminated and more through lanes and capacity can be assigned across the bridge. This type of design has been shown to extend the life of the bridge by increasing through capacity and delay the need for an immediate bridge widening project.
Disadvantages of Diverging Diamond Interchange The DDI design is a not a “silver bullet” solution for all interchanges, and there are some disadvantages to the design. There currently exists no standard design for this type of interchange, and the design is largely dependent on site specific conditions. The most obvious disadvantage is that drivers may not be familiar with this type of configuration and maneuvers due to driving on the opposite side of the roadway. Since this configuration allows one mainline movement across the bridge to operate simultaneously with an opposing ramp movement, this design and operation does not allow for free-flowing traffic of the non-freeway mainline, which we are typically accustomed to. The ramp design and intersection configuration also do not allow vehicles exiting the freeway to reenter the freeway through the road in the same direction. Instead, they must leave the interchange and turn around. This impacts drivers taking the wrong exit, vehicles looking to bypass a wreck or accident on the freeway, or large trucks bypassing a low bridge location.
Alternative Designs and Creative Solutions As engineers, we are always looking for creative design solutions to solve transportation issues. While there are many proven designs we encounter daily, it doesn’t mean that’s all we must work with. Can we think outside the box and provide creative alternate solutions to help improve roadways or solve unique traffic problems? The Diverging Diamond Interchange is a great example of a creative design solution that more transportation engineers should push themselves to explore.
About Stevie Berryman, PE
Stevie Berryman, PE is a Project Manager for Foresite Group’s Traffic Engineering Division in Peachtree Corners, GA. Stevie graduated from the Georgia Institute of Technology with a Master’s degree in Construction Management. In his spare time, Stevie is a coach for a youth travel baseball program.