system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 how do stop lights know when you are there and when to change?
gpt
Many are just dumb lights and don’t know. They are on a timer — sometimes a timer that has a different schedule for light changes at different times during the day.

Some of them do have a sensor, though. They bury wire coils in the pavement that can sense when a chunk of metal is over them. The metal in the car affects the magnetic field surrounding the sensors in the pavement, signaling that a car’s there (any big chunk of steel will do). That information is sent to a small computer in a box nearby that controls the light timing so that it knows to speed up the change because a car’s waiting (and, at certain times, this can be turned off and it will just use a timer and ignore the sensors).