A unique traffic route may be coming to an interstate new you. However, it may cause you to drive on the “wrong” side of the road. It’s called a Diverging Diamond Interchange (or DDI for short) and the first DDI in middle Tennessee opened at the end of May in Spring Hill on Interstate 65 at June Lake. More of these unique intersections are planned for the middle Tennessee area.
Created by Traffic engineer Gilbert Chlewicki, the DDI features a crisscross layout that enables traffic from both directions to temporarily switch sides of the road to navigate across an overpass and then cross back and resume the original travel pattern. Concrete dividers, stop bars, traffic signals, and extensive signage are used to direct drivers through the interchange.
While this traffic pattern may seem confusing, drivers will notice that it has numerous benefits including reducing congested road conditions and crashes. The DDI is also exceptionally economical, providing lower construction costs and demanding fewer lanes than other interchange designs.
In 2009, Missouri became the first state to install a diverging diamond interchange. According to a study performed by HDR Engineering and the Missouri Department of Transportation, the DDI in Missouri saw the following changes following the first year of operation:
- Total crashes were reduced by 46%
- Left Turn Crashes were eliminated completely;
- Rear-end Crashes were down by 29%;
- Left Turn Right Angle Crashes were down by 72%;
- Other Crashes were down by 19%
Today, there are more than 200 of these interchanges in the United States.
Tennessee’s first diverging diamond interchange opened to traffic in the city of Alcoa in December of 2010 at the Alcoa Highway/State Route 115 bypass bridge in Maryville in 2010. TDOT opened the state’s second interchange on June 30, 2015 in Sevierville on Interstate 40 at State Route 66.
On May 31, the first DDI in Middle Tennessee opened in Spring Hill on Interstate 65 at June Lake, providing those in the area with a much-needed second direct access point to the interstate, while also connecting to Lewisburg Pike on the eastern side of I-65.
As of June 2024, across the US, there are over 70 diverging diamond interchanges either in the planning stages or currently under construction. Two of those are in Middle Tennessee.
Work began on a diverging diamond interchange in Nashville in 2022 to help ease traffic congestion near the Nashville International Airport.
The flourishing city saw rapid growth in the area which caused an increased traffic volume to the previous I-40 interchange at Donelson Pike/State Route 255 at Exit 216. The current project involves relocation of that interchange and the existing state route. The newly constructed exit will be converting into a DDI.
Construction is expected to be completed by August 2027. Although in efforts to expedite the completion, the Metro Nashville Airport Authority agreed in May to provide incentives if the project is completed a year ahead of schedule.
TDOT also plans to construct a DDI at Central Pike and I-40 in Mt. Juliet. This project will also comprise the widening of Central Pike from the proposed interchange to Mt. Juliet Road/State Route 171.
Both of these projects in Mt. Juliet are scheduled in TDOT’s 10-Year Project Plan to begin construction in Fiscal Year 2030.
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