How do stoplights know when cars are present?

Answer 1

The next time you pull up to a traffic signal, look for a square shape cut into the asphalt and a line leading off of the road towards the direcion of the traffic signal control box. This is a stainless steel or aluminum box mounted on a pole near the intersection. There is a wire buried in the asphalt that senses the magnetic field of the vehicle and tells the lights when to change. In older parts of cities there is an inverted bar buried in the asphalt or concrete that is shaped live an inverted V. When the car rolls over this, it sends a signal to the same type of box. There were used in the 40's and 50's to make the lights change. Just knowing this little fact, the next time you pull up to a light trying to make a left turn, park on top of this square and it will give you the left turn arrow almost every time.