‘Diamond' interchange coming to I-285 in Dunwoody

The state’s first "diverging diamond" interchange, at I-285 and Ashford-Dunwoody, could be complete as early as September 2012, according to the state Department of Transportation.

The unusual design, known as a DDI, routes traffic in both directions to the left while crossing the interchange bridge, so that the lanes are briefly reversed. That allows cars turning left onto the interstate ramp to do so without crossing oncoming traffic. Cars coming off the interstate also can merge more easily with traffic, designers say.

The agency has awarded a $4.6 million reconfiguration contract to E.R. Snell Contractor, Inc., of Snellville. It was one of 18 project contracts awarded by the DOT, totaling more than $34 million.

The half-mile Ashford-Dunwoody project will include rebuilding the interchange and widening the bridge that crosses I-285.

“We think this is going to be something that, once motorists become accustomed to, they will really embrace,” DOT Commissioner Vance C. Smith said in a news release. “We believe DDIs are going to enable us to make meaningful, relatively inexpensive, improvements at clogged interchanges without having to expend huge amounts of money and time rebuilding the entire structures.”

He also said DDIs can be built faster and for less money. The $4.6 million reconfiguration is a fraction of a major bridge rebuild, which could cost $100 million or more.

The design is the first of its kind in Georgia and the seventh in the country, officials say. The concept was first introduced in Missouri in 2009, with others following in Utah, Tennessee and Louisiana.

Local business and government officials are also pitching DDIs at I-85 and Jimmy Carter Boulevard and I-85 and Pleasant Hill Road in Gwinnett County, and at I-75 and Wade Green Road in Cobb County.