How do stoplights know when to change?

Waiting for a stoplight to turn green can be really annoying, especially when you’re running late. It would be awesome if you had a magic wand you could wave to get the light to change. But alas, such things don’t exist. But how do stoplights know when to switch colors? We asked Jerry Kotzenmacher with the Minnesota Department of Transportation to help us find the answer.

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