Cobb County Government

Work to begin on I-75/Windy Hill diverging diamond interchange

(Dec. 31, Marietta, Ga.) -- Construction on the diverging diamond interchange project is set to begin the first week of January. In spring 2017, motorists will experience driving on the wrong side of the road at the Interstate 75/Windy Hill Road overpass.

Since the mid 1980s, a surge in population and business development has caused a steady increase in traffic in the Windy Hill corridor. A 2010 Summary of Crash and Injury Data & Comparison to Statewide Rates Report detailed crash rates along Windy Hill Road to I-75 to be three times greater than statewide crash rates. Injury rates were double statewide rates. The report also showed crash and injury rates would remain high unless corrective safety measures were taken.

Initial studies regarding how to improve safety and efficiency of the I-75/Windy Hill Rd. interchange began well before the release of the 2010 report. Planning and design of the $20 million I-75/Windy Hill DDI began after the 2010 report findings were released.

“A DDI basically eliminates the need for a left turn signal,” said Jim Wilgus, interim director, Cobb County Department of Transportation. “Eliminating that extra point of conflict eliminates traffic holdups and potentially serious crashes.”

DDIs are cost effective because they can be constructed using the existing bridge structure, eliminating the cost of building a new structure.

“It will take slightly more than a year to build the DDI,” Wilgus said. “Traditional construction takes upward of two years.”

This is one of five major projects included in the $48 million traffic and safety improvement project on a 1.95 mile stretch of Windy Hill Road from Windy Hill Road and Cobb Parkway (US-41) to east end of Windy Hill Road past Powers Ferry Road. Current projects underway include widening roads, adding medians and relocating and replacing sidewalks.

The I-75/Windy Hill DDI is funded through a variety of sources including SPLOST 2011, Georgia Department of Transportation, Cumberland Community Improvement District, Federal Highway Administration and the State Road and Tollway Authority.

When the project is completed, motorists will benefit from improved driver safety, driver efficiency and decrease congestion. By crossing eastbound and westbound travel lanes across the bridge within the interchange, the left turn lanes are provided priority at the ramp intersections reducing delays and increasing capacity.

While somewhat of a newer concept in the U.S., DDIs have been used since the 1970s in Versailles, Le Perreux-sur-Marne and Seclin, France. The first DDI in the U.S. was built in 2009 in Springfield, Mo. The first DDI built in the state of Georgia was the DDI at I-285/Ashford Dunwoody in 2011.

For more diverging diamond information, log onto For information about construction and travel times affecting your daily commute, go to