Launch Slideshow

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Exit, lane left

Exit, lane left

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    Step-by-step instructions for how to make a left turn in a displaced left-turn intersection. Images: Gresham, Smith and Partners

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    In Snellville, Ga., an innovative solution was needed that could both improve traffic flow and balance impacts to the city and property owners. Since the heaviest left-turn movement was from US 78/SR 10 to SR 124, a two-legged displaced left turn was proposed on US78/SR 10, and a free-flow right-turn lane was proposed for SR 124 to US78/SR 10.

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PUTTING THE PIECES TOGETHER

When considering whether a displaced left-turn (DLT) intersection is the best solution, consider the following questions:

  • Is there a need at this intersection that cannot be addressed with traditional operational improvements? DLTs often are viable alternatives for traffic volumes with heavy left-turning movements. However, they are generally not practical for major urban downtown areas with constrained right-of-way and railroad crossings in close proximity to the intersection.
  • Does the geometry of the intersection allow for a displaced left turn? Vertical grades should be flat to moderate and horizontal alignments 80 to 90 degrees. The road should also have sufficient storage — about 400 feet — to relocate the left-turn lanes away from the intersection. Also remember that even though the best layout might not be possible, displaced left turns could still improve traffic flow at the intersection (see “Plan B still works”).
  • Is funding available to implement a grade-separated interchange? Or do right-of-way constraints prevent a major grade separation? Compare the benefit-cost ratio of the DLT to other alternatives. The design often tends to have higher benefit-cost ratios due to significantly lower construction cost.
  • Is the design supported by the public agencies, and will the community support it? To garner agency and public support, public outreach may be needed to educate officials and constituents on the benefits of the design. A “champion” may need to promote the project within the public agency.
  • Are there other access locations? Would future development encourage new access locations? The ideal situation is if no access is needed at the four quadrants immediately adjacent to the intersection. However, access can be provided either via U-turn openings in the median for the displaced left turn or connector streets. If all four quadrants are fully built-out with no interconnectivity, then a DLT might not be the best solution.
  • Are there major pedestrian movements or transit stops at the intersection? A DLT uses multiple stages to cross pedestrians through the intersection via refuge islands, and actually reduces the overall pedestrian crossing time when compared to a traditional intersection. Therefore, it's a good option for pedestrian crossings. Preferably, transit stops should be located upstream or downstream from the intersection. High-transit traffic can be accommodated via bus pullovers.