An inductor is a coil of wire that, when energized, creates an electric field. The wire is placed in the asphalt in a loop to close a circuit and create a sensor. In the road, they look like black squares or diamonds and are usually centered in each lane. When a car rolls over the sensor, the magnetic field created by the inductor becomes more intense. A microprocessor that's located at the side of the road recognizes this change and begins the process of changing the light.