By Patrick Fox

The Atlanta Journal-Constitution

A revolutionary highway interchange design could cut delays at two notorious Gwinnett sites by as much as half.

Plans are under way to reconfigure interchanges along I-85 at Jimmy Carter Boulevard and Pleasant Hill Road to relieve the mile-long backups drivers face twice daily. The new design, called a diverging diamond, provides a freer flow of traffic by favoring left turns from the overpass onto the interstate.

If approved for construction, the interchanges would become among the first in the United States with the diamond design.

"It's working great," said Don Saiko, project manager for the Missouri DOT, which completed the nation's first diverging diamond last June in Springfield. "The commuters say it has totally reduced their time."

Another diverging diamond is expected to be completed this year in Idaho.

The Federal Highway Administration estimates the Missouri interchange accommodates 600 left turns onto the freeway per hour per lane, twice that of those with the regular design.

The diverging diamond does carry a slight "freak-out factor," according to Mike Rushing, engineer with Kimley-Horn and Associates, which has completed a study for the Jimmy Carter project.

The design allows right turns onto an on-ramp well before the overpass, but the remainder of the traffic then diverts into the left lanes at a signal light at the overpass entrance. By driving on the "wrong side" of the overpass, drivers can make an unimpeded left turn onto the freeway ramp. After vehicles cross the overpass, another signal light allows them to cross back over to the right lanes.

Pedestrians would cross the interchange in a center island.

Rushing said the design is an inexpensive fix to a chronic problem. The community improvement districts leading the drive for the projects say both interchanges could be modified without widening the bridges. Both CIDs are running parallel in the planning stages.

The projects are estimated to cost between $2 million and $3 million and could be funded with local SPLOST money, saving the time and expense of most federal and state regulations. However, Rushing said, because both Jimmy Carter Boulevard and Pleasant Hill Road are state routes, and I-85 is a federal highway, some portions of the project will have to be coordinated with federal and state authorities.

The latest traffic studies put daily traffic at about 55,000 on both roads.

Brian Allen, Gwinnett County transportation director, said the county supports anything to help relieve congestion, especially if it can be accomplished at low cost. Neither bridge is structurally deficient, he said, so the small investment now could buy another eight to 10 years of use out of the current structures.

"We want to see these move forward as quickly as possible," said Joe Allen, executive director for the Gwinnett Place CID, which includes Pleasant Hill Road. "We see this as an interim step. This is not a cure to the long-rage challenges we face with that chokepoint."