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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Similar results, cheaper solution

Project: SR 400 and SR 53 intersection, Dawsonville, Ga.
Design cost: $2.4 million
Estimated construction cost: $10 million – $12 million
Estimated construction time line: February 2015 – August 2016

Grade separation would be the best — but more costly — solution for this intersection. An overpass would allow through-traffic on SR 400 to cross SR 53 without stopping.

But limited funding led GDOT to choose a displaced left-turn design that promises similar operational improvement at 10% to 20% the cost. A two-legged displaced left turn was proposed for SR 400 with free-flow right-turn lanes on SR 53.

Fortunately, the intersection has several good design characteristics for conversion to a DLT. The intersection angle is 90 degrees, which provides adequate sight distance for drivers and allows roadway designers to widen intersection legs without having to super-elevate for roadway curvature. Plus, the existing right-of-way width and grassy median along SR 400 provides ample space to add dual left-turn lanes. The intersection's heavy left-turn movements from SR 400 onto SR 53 also make it a good candidate.

Plan B still works

Project: Jimmy Carter Boulevard/SR 140 and Buford Highway SR 13 intersection in Norcross
Design cost: $800,000 (paid for by Gwinnett Village Community Improvement District)
Estimated construction cost: $5 million – $7 million
Estimated construction time line: January 2014 – June 2015

The Gwinnett Village Community Improvement District initially considered two grade-separated interchange concepts: a compact interchange and a jug handle/quadrant road. The costs for these options ranged from $15 million to $50 million, so the community improvement district also evaluated, and ultimately approved, a displaced left-turn lane proposal as a low-cost alternative.

The proposal was not without its hiccups. The intersection warranted a two-legged continuous-flow intersection on Jimmy Carter Boulevard, but several constraints exist. First, the grade prior to the intersection is more than 10%, which would slow vehicles traversing through the DLT, and make it difficult for approaching drivers to see the displaced left-turn traffic signals. This scenario would require longer signal times that would reduce overall efficiency.

Second, a railroad overpass bridge is in close proximity to the intersection, so the crossover movement for the left-turn lanes couldn't be placed far enough away from the main intersection to ensure proper operation. Third, South Peachtree Street is the main entrance into downtown Norcross. The design would change access to the street to right in, right out, thus limiting left-turn access into the downtown area from the boulevard.

Buford Highway, on the other hand, had few if any constraints. Sufficient sight distance was available and no major impacts would prevent the construction of the left-turn crossover lanes. Therefore, even though the traffic volumes support the displaced left-turn lanes be built on Jimmy Carter Boulevard, the district decided to construct them on Buford Highway instead.

The solution still works because it improves traffic-signal efficiency and significantly reduces the delay at the intersection.

Conventional methods win

Project: Whitfield County Safety Project
Design cost: $300,000
Estimated construction cost: $2 million – $3 million
Estimated construction time line: May 2012 – May 2013

GDOT decided not to use a displaced left-turn lane because the intersection isn't a good candidate for the design. Problematic features included steep approach grades to the intersection that would hinder drivers' ability to see the traffic signals. Also, a major structure located close to the intersection would have to be widened to add a displaced left-turn lane on that leg, increasing costs by up to $1 million.

Instead, the transportation department is installing a protected left-turn phase to the traffic signal and constructing dual left-turn lanes to allow more vehicles through the intersection.

As these case studies reveal, a displaced left-turn lane is a great tool for relieving traffic congestion at intersections with heavy left-turn traffic. But it should be evaluated on a case-by-case basis against other alternatives.

— Bockisch (jay_bockisch@gspnet.com) and Shelton (scott_shelton@gspnet.com) are senior associates with Gresham, Smith and Partners.

WEB EXTRA

For videos on how displaced left-turn intersections in Gwinnett County and Snellville, Ga., work, click here.