KENNESAW — Cobb’s first diverging diamond interchange is planned for Wade Green Road in Kennesaw and slated to open in late 2014, though county leaders hope to speed that up.
Cobb Commissioners got a preview of the DDI this week from Jeff Lewis, a member of the State Transportation Board. They are expected to give final approval to an agreement with GDOT for the $2.6 million interchange later this summer.
A DDI eliminates a left-turn signal for vehicles accessing freeways, and traffic planners say it is designed to increase capacity and decrease congestion at freeway interchanges. The actual interstate is not altered.
After the technology was introduced in France, Springfield, Mo., was the first city in the United States to get a DDI, in 2009. Last month, the first one in Georgia opened at Ashford-Dunwoody Road over Interstate 285 in DeKalb County. Two others are planned for Gwinnett County.
Dan McDuff, deputy director of Cobb DOT, said the Wade Green project will cost about $2.65 million, with $1 million coming from GDOT for construction and the remaining $1.65 million from the county’s 2011 SPLOST, which will go toward engineering and any additional construction costs. The existing interchange will be retrofitted, and no new lanes will be built.
“This joint venture will ensure timely delivery of the project, cost efficiency and improved mobility for the county, as well as the metro region,” Lewis said.
After allowing traffic to enter the freeway from the right lane, a signaled intersection switches traffic from the right to the left side of the road, where drivers can turn left to enter the freeway without crossing oncoming traffic. Vehicles continuing on the access road then encounter another intersection that switches traffic back to the right side of the road.
Construction of the diverging diamond interchange saved over $100 million over the cost of building a new interchange at Ashford Dunwoody, and is expected to provide congestion relief for at least 10 years, based on traffic growth projections, Lewis said.
At the Wade Green Road interchange, GDOT projects a 60 percent reduction in the amount of time drivers have to wait at traffic lights, along with a 21 percent reduction in overall morning travel times and a 13 percent decrease in evening travel times.
“We know that improved mobility can lead to less commuter frustration, and, something the feds like to hear, can lead to improved air quality,” Lewis said.
An average of 36,000 vehicles per day cross the Wade Green Road bridge, McDuff said. Around 15,000 vehicles will enter Wade Green Road from I-75 northbound, while another 15,000 will enter I-75 southbound. Meanwhile, 5,800 vehicles per day will head north on I-75 from Wade Green Road, while 4,800 driving southbound on I-75 will enter Wade Green Road.
While drivers entering or exiting I-75 can see delays of around a minute in peak times, McDuff said that doesn’t take into account the backups caused by the Wade Green Road interchange. While research hasn’t been done on the entire corridor, McDuff said delays back to Jiles Road can easily reach 10 minutes in the morning.
“When you open this up, it’s going to open things throughout the corridor,” he said.
The Ashford Dunwoody-I-285 interchange, one of 13 diverging diamonds to open in the United States, opened to traffic a month ahead of projected date, Lewis said, and has already resulted in fewer crashes.
The interchanges are considered safer because they eliminate potential crossing conflicts between vehicles turning left onto the freeway while facing opposing traffic, Lewis said. A conventional diamond interchange has four crossing conflicts.
Northeast Cobb Commissioner JoAnn Birrell, whose district includes the area of the proposed interchange, said the project will also benefit the cities of Kennesaw and Acworth. She compares the diverging diamond to roundabouts, another concept that gained popularity in Europe before coming to the United States.
“It’s going to probably be confusing at first, but the drivers will be helped by the islands and the directional signage that make it easier to understand,” Birrell said. “It’s going to be a big benefit down the road.”
GDOT spokesman Mark McKinnon said that at the Ashford Dunwoody interchange, the agency had an extensive public education campaign on the project.
“The signing and the marking makes the expectations pretty intuitive,” he said.
While Cobb DOT has yet to work out the details with GDOT, McDuff said public involvement will be a likely component of developing the interchange.
Commission Chairman Tim Lee also supports the interchange.
“It will be a significant improvement to traffic,” he said. “It will help traffic flow. It will help safety and reduce congestion.”
And more could be coming.
“We’ll evaluate all our interchanges like that to see if that kind of technology will help,” Lee said.
Northwest Cobb Commissioner Helen Goreham said she has yet to see the Ashford Dunwoody-I-285 interchange, but she has heard good things about the diverging diamond interchange.
“Following the news on it, it sounds like it’s fabulous,” she said. “Anything that can help the flow of traffic is wonderful.”
Southeast Cobb Commissioner Bob Ott said the county made sure in the 2011 SPLOST to leave open the possibility of allowing for new technologies to be incorporated. In previous SPLOSTs, rigid language meant the county was locked into a particular type of project, even if the technology changed over the life of the SPLOST.
“What you’re seeing is that they are taking some ideas that worked in other places and bringing them here,” he said.
The development process could delay construction until late 2014, though McDuff said the county is working with GDOT to speed the process up. Once started, construction is expected to take between six and eight months, with the existing interchange remaining open during construction.
McDuff said the Wade Green Road-I-75 interchange, which will also include construction of sidewalks, made sense for the first diverging diamond partly because it won’t require the county to buy new right-of-way.
“The existing bridge will accommodate the proposed interchange, and there will be minimal disruption to businesses in the corridor,” McDuff said. “Also, analysis of the traffic at this location makes it a great candidate with significant reductions in delay after construction.”
Cobb has been discussing the DDI with GDOT for about six months as part of talks to improve congestion at the interchange, McDuff said.
McKinnon said Cobb DOT came to GDOT to propose improvements at Wade Green Road, just as the Perimeter CID initiated discussions on the Ashford Dunwoody Road interchange.
The DDI is the latest road project announced for the area near Kennesaw State University. To the south, the extension of Big Shanty Road will have a ribbon-cutting next week, while the planned bridge connecting Busbee Drive and Frey Road, which is still seeking funding, is expected to provide traffic relief at Chastain Road and I-75.