McDonough DDI undergoes late revisions

Aerial photo of I-75 exit 218 / state route 20 (staff photo)
I-75 exit 218 / state route 20 in McDonough (staff photo)

A late change in concept has altered the upcoming plans to construct a diverging diamond interchange in McDonough. The project, located at I-75 exit 218 / state route 20, is scheduled to accept bids for construction in July 2020.

The previous concept for the DDI featured seven lanes, including two through lanes in each direction, one left-hand turn lane from state route 20 westbound to I-75 southbound, and two left-hand turn lanes from SR 20 eastbound to I-75 northbound — mirroring the existing lanes there today. To widen the outside shoulder from two feet to four feet, and to increase the lane width from 11.2 feet to 12.33 feet, the second left-hand turn lane on state route 20 eastbound has been removed.

The new plans include two through lanes and one left-hand turn lane in each direction over the bridge. Right-hand turn lanes are located prior to the bridge structure and therefore not referenced.

The updated cost estimate for the project is $5.8 million, including $5.1 million for construction. DDI installs cost a fraction of more traditional interchange expansion projects, which can often be in the range of $25–75 million dollars.

Construction is estimated to last 10–12 months once started.

What is the DDI?

The diverging diamond interchange is an innovative roadway design that improves traffic operations by 30 – 40% while also reducing crash rates. The design originated in France during the 1970s and was first built in the United States in 2009.

The design utilizes a crossover, where vehicles travel on the left-hand side of the roadway within the interchange. As drivers approach an interstate exit, they crossover to the left-hand side at the first traffic signal within the exit. After traveling over the bridge, they cross back to the right-hand side in the second traffic signal. This allows left-hand turning vehicles to enter or exit the interstate without the need to turn in front of opposing traffic. Additional road signage, lane markings, and concrete medians help motorists navigate the interchange. Most drivers grow accustomed to the design after only a few trips through the exit.

A visualization is available from North Carolina DOT; I recommend this video for anyone wanting a visual aid to help understand the roadway design.