By Jay Bockisch, PE, PTOE, and Scott Shelton, PE
In better times, grade separation was the cure-all solution for intersections experiencing traffic volumes significantly over their design capacity. By adding an interchange, traffic engineers and roadway designers improved traffic flow, reduced driver delay, and improved safety. However, today's tight budgets, limited rights of way, community opposition, and increased vehicle-miles traveled are compelling public agencies to find lower-cost and less obstructive alternatives to a grade-separation interchange.
One alternative being implemented with increasing frequency is the displaced left-turn (DLT) design, also known as a continuous-flow intersection (CFI) or crossover displaced left-turn intersection (XDL). Instead of constructing tunnels, ramps, and/ or bridges that are normally needed for a grade-separation interchange, a DLT incorporates traffic-signal timing, turn-lane placement, and access control to increase capacity at an intersection. The construction cost is approximately 80% to 90% less than a grade-separated interchange, while providing approximately 75% of the capacity improvements of grade separation.
Before you decide if this design is the best fit for your intersection, you must first understand how it works and when it is deemed most successful.
What is a DLT?
Although displaced left-turn designs look complicated on paper, they operate just like a traditional intersection. The differences begin with the traffic flow of left-turn and through-movements. At a typical intersection, the left-turn movement of vehicles proceeds prior to the through-movement occurring. A displaced left-turn design allows left turns to cross the opposing through-movement at a signalized intersection approximately 400 feet before the main intersection.
After crossing the opposing through-traffic lane, left-turn vehicles travel on a roadway parallel to the opposing lanes, then execute the left turn simultaneously with the through-traffic at the main intersection. Traffic signals at the left-turn crossovers and the main intersection are operated in a coordinated mode so vehicles do not stop multiple times in the intersection area.
Ideal for alleviating congested intersections with heavy left-turn traffic, DLT designs can help improve operational efficiency by limiting the number of traffic-signal phases, and can potentially reduce crashes by decreasing the number of conflict points at the intersection. They're typically constructed within 18 months, with traffic maintained on the existing roadway as widening occurs to the outside. In contrast, a grade-separated interchange typically takes 24 to 36 months and costs up to $30 million to construct.
The following case studies explain why Georgia DOT (GDOT) decided to implement this intersection design at three locations — and decided against it at another.
Satisfying constituent concerns
Project: US 78/SR 10 and SR 124 intersection, Snellville, Ga.
Design cost: $2.5 million
Estimated construction cost: $7 million – $10 million