If you live in the Kansas City or Overland Park areas, you have probably driven through a diverging diamond interchange (DDI). In Lenexa, shoppers going to the Oak Park Mall have been crossing through a DDI over I-35 since it opened last summer. Open for two years now, the DDI at Roe and I-95 gives people easier access to Town Center Plaza stores.
These innovative new intersections may have thrown drivers for a loop when they first encountered them, but data shows they increase safety and improve the flow of traffic in the long run. Like roundabouts, now common in the Kansas City area, the diverging diamond highway interchange is taking root and drivers should know more about them.
Some Background on the New Design
The diverging diamond interchange was first proposed in the U.S. by a graduate student in transportation engineering at the University of Maryland in 2000. Following an intensive study of the design by the Federal Highway Administration (FHWA), the first such intersection in the U.S. was completed in Springfield, MO in 2009. There are now over 60 DDIs across the country with at least as many more in the design or construction phase. There are 10 DDIs in the Kansas City/Overland Park area alone—the most of any urban area. So, why the popularity?
How Diverging Diamonds Work
Whether you're entering or exiting a highway in a congested urban area or simply crossing over the highway, traditional intersections can become traffic nightmares during peak travel hours. At a traditional interchange, drivers entering the highway in one direction must make a left turn across oncoming traffic, as must drivers exiting the highway. This creates the potential for traffic back-ups and a risk of dangerous side-impact collisions
With the diverging diamond interchange, all of the travel lanes on the crossroad pass through a traffic signal before crossing to the “wrong” side of the highway. Before crossing over, cars have the opportunity to merge onto the highway on the right side. Once the lanes have diverged, cars entering the highway on the left can simply merge on as they would with a right-side entrance. Cars not entering the highway stay in their lanes and cross back after passing over (or under) the highway.
While the process may be confusing to read, in practice, drivers simply follow the signs as they would with any highway entrance. The advantages to a DDI include the following:
- Fewer conflict points. While there are 26 possible points of conflict in a conventional interchange, there are only 14 in a DDI and they are spread throughout the interchange, rather than concentrated in a few dangerous spots.
- Better sight distance at turns. Since the DDI eliminates cross-traffic turns, drivers have better visibility as they merge onto and off of the highway.
- No driver confusion. The FHWA study and observations of the Springfield DDI indicate that even drivers who have never encountered a DDI manage to navigate it with virtually no confusion.
- Traffic calming features when desired. Because traffic is controlled by traffic signals, the pattern can be altered as needed. For example, when a nearby sporting event or concert is letting out, lights can be timed to allow for easy exit.
- Eliminates wrong-way entrance to highway. Believe it or not, drivers entering a highway in the wrong direction is a common problem. The DDI makes it difficult—if not impossible—to make this mistake, saving lives as a result.
- Fewer crashes. Most importantly, data on existing DDIs indicates a 60 percent reduction of collisions over conventional interchanges.
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