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Traffic signals use sensors and/or are programmed

Some traffic lights are triggered by approaching traffic. Does Medford use more pressure plates in the road or some sort of optical sensor?

— Theresa P., Medford

Theresa, some of the traffic signals use sensors, some are programmed, and some are programmed for part of the day and rely on sensors for the rest, depending on the traffic volume.

You may have noticed the circles or diamonds in the road near some intersections. These “loops” are actually copper wires that pick up magnetic anomalies, explained Public Works Director Cory Crebbin.

Sometimes “advanced loops” are placed further from the intersection to detect a car as it approaches, and, if there are no other cars waiting, the signal controller gives the car a green light by the time it reaches the intersection.

However, the city of Medford is moving away from loops because it makes roadwork difficult and, instead, is installing video-detection equipment. Unfortunately, the video equipment may have problems detecting fog-colored cars on a foggy day, Crebbin said.

"But that does not happen very often,” he said.

Signals at intersections with higher traffic counts are programmed. The city’s Engineering Department calculates the timing of the lights based on a complicated math formula that we don't want to bore you or our other readers with.

There are different types of programming. For example, Highway 62 uses “adaptive signal timing” so the signals up and down the corridor are communicating with each other and adapting to get as many cars through as possible.

“So if you time things perfectly and you’re driving the speed limit or less, you’ll hit all green lights,” Crebbin said, adding that the city plans to switch to adaptive signal timing on Barnett and McAndrews roads in the future.

The signals on Central Avenue and Riverside Avenue are synchronized so the lights are set on a timer regardless of the traffic volume.

“The problem with doing this for north and south is then you can’t do it for east and west because there is no math solution for that,” Crebbin said.

Send questions to “Since You Asked,” Mail Tribune Newsroom, P.O. Box 1108, Medford, OR 97501; by fax to 541-776-4376; or by email to youasked@mailtribune.com. We’re sorry, but the volume of questions received prevents us from answering all of them.