Highway Improvements on a Budget: Diverging Diamonds

Diverging diamond interchanges are a cost-effective way to decrease congestion and increase safety at intersections. What is a diverging diamond interchange? The two directions of traffic cross to the opposite side of the road. This occurs on both sides of the bridge at the freeway intersection.

Originally developed in France, the first diverging diamond interchange in the U.S. was built in 2009 at the intersection of SR 13 and I-44. Kentucky, Tennessee and Utah have also installed the interchange. Three such conversions are underway in metro Atlanta.

There are three advantages of diverging diamond interchanges. First, the design improves the efficiency of the interchange. As there are only two clearance intervals (or phases) compared to the typical six, all directions of traffic have a higher percentage of green-time at traffic lights. According to FHWA, during high traffic times DDI interchanges can handle between 100-400 more vehicles per hour than a conventional interchange. Delays during heavy traffic are decreased between 10 and 40 seconds.

Second, diverging diamonds are cost-effective as they more fully uses existing land space. In Missouri, the interchange cost $3 million compared to the $10 million for a new bridge. Virginia expects a diverging diamond on US 460 to cost 50-75 percent less than a traditional diamond. Diverging diamonds can be constructed in about 25% of the time of traditional diamonds. Additionally, diverging diamonds reduce fuel consumption by 80 percent.

Third, diverging diamonds will likely decrease the number of crashes. In Missouri, the number of crashes decreased by 60% between 2008 and 2009. FHWA found that drivers maintained their lane through the intersection. The study further found that due to speed and conflict point reductions, any crashes would be less severe than in traditional diamond interchanges. Additionally, the number of navigation errors for a diverging diamond was slightly less than for the traditional diamond interchange. While it may be counterintuitive, one of the biggest advantages of a diverging diamond interchanges is the improved safety.

In some situations, diverging diamonds are not a permanent solution. Three locations in Georgia are installing diverging diamonds as temporary solutions. A more permanent solution of converting a diamond interchange to a turbine interchange costs $50 million. This is money the state does not currently have. However, the diverging diamond can provide an effective interim solution at a very attractive price.