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
Estimated construction time line: March 2014 – October 2015
A well-known traffic problem to local commuters, the intersection's heavy left-turn movements in the morning and afternoon rush hours slow traffic to a crawl. Design and consulting firm Gresham, Smith and Partners performed an intersection capacity analysis that graded the intersection's level of service with an E and gave operational efficiency an F (E/F; intersections are given a letter grade to describe service level/operational efficiency with an A representing very good and F as very bad).
The morning rush-hour intersection delay is 60 seconds and the afternoon delay is 126 seconds. The firm projected that in 20 years the level of service will drop to F/F with an average delay of more than three minutes.
GDOT considered many options to improve the intersection, from building a bypass to divert traffic around Snellville to building a multilevel traffic circle estimated to cost $100 million. All were met with heavy opposition from the public, due to impacts to property owners.
Another solution was to grade separate the intersection to provide more efficient through capacity and to separate the left-turn movement from the through-movement. This solution wasn't considered viable for several reasons. First, the intersection is in the heart of downtown and grade separation would sever the city in two and might potentially create a high-speed corridor through the city. Second, several historic houses exist along the roads leading to the intersection, and the project would demolish these resources and jeopardize any federal funding. Third, the intersection is on a curve so US 78/SR 10 would have to be straightened to accommodate bridge construction. The overall cost was estimated to be approximately $80 million.
GDOT needed a different solution that could both improve traffic flow and balance impacts to the city and its property owners. Since the heaviest left-turn movement is from US 78/SR 10 to SR 124, a two-legged displaced left-turn lane was proposed on US 78/SR 10, with a free-flow right-turn lane for SR 124 to US 78/SR 10. The estimated construction cost is approximately one-tenth that of a grade separation.
The two-legged displaced left turn is being constructed by widening the northern side of the intersection, thereby protecting the historic resources on the south side and reducing the overall construction cost by $3 million to $5 million. This will allow the intersection to operate at a C/D level of service, with a 33-second delay in the morning and 38-second delay in the afternoon.
An underutilized bypass around the intersection to the south will be used as a detour during construction, and will serve in the future as a bypass for through-traffic on US 78/SR 10. This will further improve the level of service.