Diagram courtesy of Gilbert Chlewicki http://www.divergingdiamond.com
DDIs are a relatively recent development in highway construction. This innovative form of interchange was developed in 2000 by Gilbert Chlewicki, who explained that this is “the
“Arrive, Crossover, Drive” will be the new catch-phrase beginning this summer when traveling through the Interstate 285 Interchange at Ashford Dunwoody Road in the City of Dunwoody in Metro Atlanta, Ga.
So said Yvonne Williams, president and CEO of the Perimeter Community Improvement Districts (PCIDs), a not-for-profit organization that has been working for the past 12 years to improve mobility and access in the Perimeter market through transportation infrastructure improvements. Commercial property owners within the PCIDs voluntarily pay additional property taxes to initiate and help implement these improvements.
To this end, the PCIDs and the Georgia Department of Transportation (GDOT) have launched a Diverging Diamond Interchange (DDI) project as an interim solution to the congestion at the I-285-Ashford Dunwoody Interchange until a complete reconfiguration of the interchange can be made.
The PCIDs initiated Georgia’s first such interchange when they hired Moreland Altobelli Associates Inc, Norcross, Ga, to conduct pre-engineering studies and find a new concept for the 43-year-old interchange, a major gateway to Central Perimeter, one of the largest employment centers in Metro Atlanta. When I-285 was opened in 1969, much of Perimeter was still farmland. At present 123,000 people work in Perimeter each day and approximately 90 percent of those commute to the area. In addition, 80 percent of the almost 67,000 people who live in Perimeter commute out of the area to work. Thus the need for improvements to handle the growth.
The PCIDs received $450,000 from DeKalb County, where Dunwoody is located, and an $800,000 grant from the Georgia State Road and Tollway Authority for the pre-engineering and project design.
Moreland Altobelli Associates Inc. ultimately recommended the diverging diamond interchange. Buddy Gratton, president of the company, noted that Moreland Altobelli and the PCIDs actively coordinated with the GDOT and the Federal Highway Administration, “which allowed the project to move from idea to concept to construction in record time.”
DDIs are a relatively recent development in highway construction. This innovative form of interchange was developed in 2000 by Gilbert Chlewicki, who recently joined Wallace Montgomery & Associates LLP, a Towson, Md., based consulting firm. Wallace Montgomery is viewed to be among the top innovative transportation engineering firms specializing in the design and construction of highways, bridges and traffic facilities.
“The DDI works by moving traffic above [or below] the freeway on the left side of the road. As the arterial traffic approaches the interchange, there will be a signalized intersection that crosses the opposing thru traffic so that on the bridge, people will be driving on the left side of the road. After crossing the freeway, there will be a second signalized crossover to bring traffic back to the right side of the road,” Chlewicki explained.
“The great benefit of the design is that left turns now become direct turns, just like right turns usually are. Because of this, we don’t need an extra signal phase for the left turns [which can improve traffic operations] and we don’t necessarily need left turn lanes on [or under] the bridge [which reduces the bridge structure footprint],” he went on.
Chlewicki described the DDI as a compact design that can, in the correct situation, be constructed with a smaller bridge.
“This really reduces the construction cost while at the same time improving traffic operations and road safety,” he pointed out, adding that the DDI “is most beneficial when there is at least one left turn movement that is heavy, but it can be beneficial in other situations as well.”
The Dunwoody interchange is designed to significantly improve traffic flow and safety without constructing a new bridge. Restriping, altered signal timing, improved turning conditions, and a reconstructed ramp will be used instead to guide and safely separate opposing traffic as it moves through the interchange, allowing traffic to flow faster and reducing accidents due to fewer traffic points of conflict.
The traffic on Ashford Dunwoody Road briefly crosses from the right side of the roadway to the left side at a ramp intersection signal so vehicles may make free left turns onto the I-285 freeway on-ramp. Traffic crosses back to the right side of the road at a second ramp intersection. A loop ramp used to access I-285 East from the interchange bridge will be removed as a significant contributor to the congestion in this corridor. A sidewalk for pedestrians is being installed in the bridge median inside a concrete barrier-protected pedestrian only zone.
As a result, according to the PCIDs’ Williams, it is expected that under normal, free-flowing traffic conditions on surrounding highways, the I-285 and Ashford Dunwoody DDI will reduce traffic delays in evening rush hours up to 20 percent.
“There are significant safety improvements also with DDIs. Total crashes were down 46 percent in the first year of operation of the first DDI in the country in Springfield, Mo., which opened June 1, 2009. Left turn type crashes were eliminated and left turn right angle type crashes were down 72 percent because of how left turns are handled within the DDI,” she noted.
The project also involves new plantings at the interchange. Efforts have been made by the organization to find homes for the mature crape myrtles and other trees at that location, and some of the existing landscaping, particularly perennials, was replanted in the area.
E.R. Snell Contractors Inc., Snellville, Ga., is overseeing the state-funded $4.6 million project, awarded in August 2011 with an estimated completion date of Sept. 30, 2012. Construction began in January 2012.
The public was able to test drive the DDI before the crossover of traffic begins. The PCIDs organization sponsored an event on May 19 in the parking lot of Perimeter Mall, during which drivers used golf carts to drive on a simulated track for a “windshield level” experience of the new interchange.
Summing up, PCIDs’ Williams described the Dunwoody diverging diamond interchange as “a proven, cutting-edge concept for providing an interim, immediate, and cost effective reduction in traffic congestion in a major gateway into the economically important Perimeter market.
“Strategic upgrades in core transportation infrastructure are critical to sustaining economic growth and development in Central Perimeter, Metro Atlanta’s dominant office market. This economic health is important also to local and state governments that depend on the tax revenues generated in Perimeter,” she added.
There are currently only 11 of this type of interchange open to traffic in the United States, with that number anticipated to at least double by the end of next year. The Georgia Department of Transportation expects the Dunwoody DDI to become the model for congested interchanges throughout the state.
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