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Why do stoplights for streets that seem similar have such different timing?

Ken Little

Q. Why do stoplights for streets that seem similar have such different timing? I notice when a vehicle is exiting Arboretum Drive in Landfall the light changes right away, but when a vehicle is stopped at the lights at Station Road and Covil Farms Road they have a significant wait time.

A. While the signals are from the N.C. Department of Transportation, they are within the city of Wilmington’s signal system, which oversees the maintenance of the timing and equipment, said Jessi Leonard, Division 3 traffic engineer.

Wilmington’s signal systems management engineer, Denys Vielkanowitz, offered this answer to the question:

“As with most things in life, there are many variables, one of which is a little luck,” he wrote.

“Nearly all of the signals in the Wilmington area are programmed to run ‘cycles,’ or the same thing over and over again, exactly like clockwork, for the majority of the day and evening to accommodate expected traffic volumes. Some cycles are long for very busy traffic time periods while some cycles are shorter to accommodate less busy traffic time periods,” Vielkanowitz wrote.

He gave a “generic example”:

“For descriptive purposes, consider the face of a clock as a ’60-second cycle’ with 12 being the ‘top of the cycle.’ Also to simplify, let’s say the ‘main street’ in this example is Military Cutoff Road while the ‘minor street’ or ‘side street’ is Arboretum Drive,” Vielkanowitz wrote.

“Let’s assume the main street is programmed for 40 seconds and the side street is programmed for 20 seconds. The sum of those values equals the cycle that is running (60 seconds). That means that at the 12 position on the clock face (the top of the cycle), Military Cutoff Road will be active until the 8 position on the clock face (40 seconds). Then, Arboretum Drive will be active from the 8 position until the 12 position on the clock face (20 seconds). This will repeat as long as this cycle length is scheduled to run.

“So, one answer to your question is related to ‘when’ a vehicle arrives on the side street … the luck factor. Consider if a vehicle pulls up on Arboretum Drive at the 12 position in the above example. That vehicle would have to wait 40 seconds before getting service. Alternatively, if that same vehicle pulled up at the 7 position, they would have only waited 5 seconds,” Vielkanowitz wrote.

“Please understand the example above is generic, overly simplified for descriptive purposes and does not represent the exact operation of this specific location. There are many other variables that affect how much time each direction gets and when they get it, but the general concept of the ‘cycle’ holds true.”

Vielkanowitz wrote that another item he frequently observes “is when vehicles pull too far forward at the intersection when waiting for service.”

“There is a wide, white line painted across the road called a stopbar or stop line. The proper stopping location for vehicles waiting for service at a signalized intersection is completely behind this stopbar. If a vehicle crosses this line while waiting for service, the traffic signal may think the vehicle has left the intersection and decide it unnecessary to service the side street,” Vielkanowitz wrote.

Vielkanowitz included a public service announcement prepared by the city of Wilmington that explains the stopbar and proper stopping location:


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