How do traffic lights know when to change color?
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The lights are primarily controlled by wires in the pavement called “loops”. These are magnetic sensors that detect when your car drives up and that starts the clock to change the light. This can be very frustrating for motorcyclists because lots of them aren’t big enough and have enough metal for the loop to detect them.
At the most simple level, timers.
A step above that are sensors. The black circles before the light where cars stop sense the change in an electrical current when a large mass of metal like a vehicle is over it. This allows a light to sense when a car is waiting.
With this you can implement some basic programming like “if there is a car in the turn lane, then turn lane gets the green arrow before straight. If no turn lane, prioritize straight traffic.”
At a very high level there are networked lights with central control that can impact traffic flows of entire cities by changing the timing of lights.
Things like turning on freeway on ramp lights, prioritizing certain roads, holding some lights longer so that traffic ahead has time to clear, and so on.
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