Traffic Control Out of the Box- The Continuous Flow Intersection

Throughout Georgia, significantly increased traffic volumes combined with limited funding has motivated transportation engineers to develop new options to improve traffic flow. Diverging diamond and Continuous Flow Intersections are two of those options.

At first glance, the operational aspects of these intersections can be confusing, and even after viewing animation videos, it can be difficult to envision how these intersections will improve traffic flow. That’s especially true of the Continuous Flow Intersection (CFI) because the name is a bit of a misnomer. “Continuous flow” pertains to straight-through and left turn traffic (traveling in the same direction) flowing at the same time. In a conventional intersection, such as the one at Highway 78 and 124, the light turns green for traffic turning left before it turns green for vehicles traveling straight-through the intersection. This results in extended “red light” times as vehicles traveling through the intersection must wait until traffic turning left clears the intersection before getting a green light.

A CIF addresses this problem by adding left turn lanes on the left side of the roadway which allows left-turning vehicle to cross the highway before they get to the actual intersection. Since they are already on the left side of the roadway, they can pass through the intersection at the same time as vehicles traveling straight through it. That reduces wait time at the intersection. Additional lanes on the right side of the roadway reduce traffic stack-up by enabling vehicles to get out of the main flow of traffic well before they approach the intersection.

As an example, a driver traveling west on Highway 78 and planning to turn right on 124 will move into the right turn lane a few hundred feet before reaching the intersection. If traffic flow on 124 permits, the driver completes the turn and merges into the northbound lanes. During times of heavy traffic flow on 124, the driver waits in the right turn lane until the turn can be completed. To prevent traffic back-ups when volume is heavy, the 78/124 CFI will have two right turn lanes on Highway 78 heading west and Highway 124 heading south.

The CFI is primarily a project of the Georgia Department of Transportation (GDOT). The agency’s engineers determined that it was the best means of improving traffic flow at the 78/124 intersection, given existing budgetary and geographic constraints. A “flyover” intersection, such as the one at Highway 78 and Park Place in Stone Mountain, would in fact offer continuous flow, however, such an intersection is exponentially more expensive, consumes considerably more land and, for a number of reasons, is inappropriate for the middle of a city like Snellville.

Of course, the CFI won’t have the desired positive impact on traffic flow if there are back-ups at the intersections on both sides of it. That will be addressed through an updated system of traffic light controls and will be addressed by both the Georgia and Gwinnett County departments of transportation. Federal, state and county agencies are responsible for various sections of the highways that cross in the center of Snellville, so all are involved in the planning, funding and administration of the CFI initiative.

For its part, the Snellville city government doesn’t have much influence over the actual construction and design of the intersection. But by being actively involved in the planning process, it can make sure that execution is as non-invasive as possible, and that the intersection blends well with the Town Center plan.

The city is fortunate that both GDOT and county DOT representatives are not only open to discussion, but are encouraging it. At a meeting on November 14th, attended by all members of the Snellville City Council, it was apparent that GDOT is dedicated to completing the project on schedule, within budget and with an eye toward accommodating reasonable requests from the city.

The CFI is unquestionably the creation of “out of the box” thinking and while it will take a bit of getting used to, it will dramatically improve the flow of traffic at the 78/124 intersection. And that’s good for Snellville.

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