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Good Question: How Do Our Traffic Lights Work?

MINNEAPOLIS (WCCO) -  When we're stuck at a red light, it can seem like there's no rhyme or reason to how our traffic signals work. But, there is a huge system in place with the goal of shortening the time we all, collectively, spend on the road.

So, Jennifer from St. Paul wants to know: How do our traffic lights work? Good Question.

"It's important to understand we're talking about the entire system," said Klayton Eckles, director of engineering and public works for Woodbury. "We're not trying to optimize an individual's experience."

There are several different ways traffic lights can function, including detectors and timers. In Minnesota, 90% of the lights use some kind of detector to alert the controller – the large box with a computer at each intersection -- a vehicle is waiting. That doesn't necessarily mean, though, the light will turn green right away.

"We have to accommodate side street traffic as well," said Steve Mosing, traffic operations engineer for Minneapolis. "There might be a walk signal up, so we can't just cut that short. We have to wait for that to time out to give this direction green."

One type of detector is video, where the camera recognizes a change in pixels as a vehicle or bicycle and alerts the controller something is stopped at the light. Another kind of detector is an inductive loop built into the roadway pavement hundreds of feet before the light. That loop emits an electrical flux, which a vehicle breaks when it pulls up.

"It's not weight based, but motorcycles are a challenge," said Mosing. "To break the electrical flux, you need metal and cars have more metal that motorcycles."

Lights can also work is through a timed system. All of the lights in downtown Minneapolis are set on a timer, whose times differ depending on whether it's AM rush, PM rush or off-peak.

"The volumes in down Minneapolis are so consistent that we're able to time the lights based on volume," said Mosing.

During rush hour in Minneapolis, the full light cycle -- beginning of green until the next beginning of green -- is usually 110 seconds. During off-peak, it runs between 55 to 90 seconds. Traffic engineers also have special patterns for Twins or Timberwolves games and other special events.

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