How do Signal Lights work?

What makes them turn red or green?


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What Guys Said 2

  • Some of them works on fixed timers.
    Some have variable timers with an electromagnetic sensor laid out just before the stop line. The sensor detect if there's any vehicles waiting for the light to change. For those kinda traffic lights you usually see a buried wire just before the stop-line.

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  • Seriously? These days they are computer controlled. In large cities, they are centrally controlled to move traffic more smoothly. Before electronics they were controlled by a mechanical timer with adjustable contacts.

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