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RALEIGH – The N.C. Department of Transportation is piloting a first-in-the-nation design that will improve the traffic flow at busy intersections in Cary and Clayton.

The NCDOT is calling the unique design a dynamic left turn intersection, which will vary the number of available left-turn lanes using dynamic signal phasing to reduce travel delays. The design will work the same at both locations, using a pair of LED-powered overhead signals that activate or deactivate the rightmost left-turn lane, depending on the time of day and traffic conditions.

The department and the town of Cary are installing a dynamic left-turn approach for eastbound Tryon Road drivers turning left onto Cary Parkway, heading toward U.S. 1/U.S. 64. In Clayton, the NCDOT will install a dynamic left-turn approach for drivers on U.S. 70 Business westbound at Town Center Boulevard turning into the Walmart-anchored shopping center.

The Cary system has been rescheduled to begin operating Feb. 14, if the weather is dry, while the Clayton installation will happen later this spring.

During peak travel periods, such as rush hour, both left-turn lanes will be active. Left-turning vehicles will turn only under a protected green arrow, as they do now. During these times, a pair of overhead signals will display a white left-turn arrow over a black background to indicate the rightmost left-turn lane is active.

Most of the day, there is not enough traffic going left to require the use of both left-turn lanes. During those slower times, the rightmost left-turn lane will become deactivated and the overhead signal for that lane will display a red “X” over a black background. The leftmost left-turn lane will remain active, and drivers will receive a flashing yellow arrow, allowing them to turn left when it is safe to do so. A protected green arrow will still be displayed occasionally to reduce any backups in that lane.

The NCDOT has created this short instructional video and visualization and and this diagram that illustrates how the design will work.

Joe Milazzo, the executive director of the Regional Transportation Alliance, approached the NCDOT last year with the concept, and Joe Hummer, the department’s state traffic management engineer, identified two ideal locations to deploy the design. The alliance is a regional business coalition that seeks to advance transportation solutions in the Raleigh-Durham-Research Triangle region.

“We are using standard devices to reduce travel delays in a novel way,” Hummer said. “We are basically freeing up this intersection to give more green time for people going straight through it.”

The leftmost turn lane has a better view of oncoming traffic, so the dynamic signal will deactivate the rightmost left turn lane when displaying a flashing yellow arrow for drivers during lower traffic-volume periods.

Changeable message boards will alert drivers at each location in advance of the new system being activated. The NCDOT will monitor both locations to help ensure drivers adjust smoothly to the dynamic left turn lanes. The department over the next several months will evaluate the effectiveness of both locations, and the agency may deploy the dynamic system at other locations in the future.