Why do traffic lights automatically change when I pull up to them?

There are several types of sensors, but 90 percent use inductive loops.

Why do traffic lights automatically change when I pull up to them?

To improve traffic flow, many cities and towns have installed electronics in the road or mounted near the traffic light that sense the presence of your car and signal the light to change from red to green. There are several types of sensors, but 90 percent use inductive loops, says Floyd Wokmon at the California DOT.

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.

Consecutive loops in a lane can tell the processor how long to remain green depending on the number of cars in the lane.