Book 25, Question 26
How do traffic lights know when cars are waiting?
Many intersections have a loop of wire buried in the pavement.
A small electric current runs through it, and when a car sits on top, the metal changes the loop’s magnetic field, which tells the light’s computer that a vehicle is waiting. Other lights use cameras or radar. This lets the signal change when needed instead of using a fixed timer. Engineers also time lights along a street so cars at the speed limit meet green after green.