Traffic lights detect vehicles using a variety of methods.

These devices generally use some sort of transmitted energy such as radar waves or infrared beams to detect vehicles passing over the roadway.

Magnetic field sensors detect vehicles passing over them and not necessarily stationary vehicles.

In cars, for example, the chip might be used to detect other vehicles looming in the fog.

These sensors detect both vehicles and people but are prone to false alarms from animals.

A sophisticated cruise-control function would keep the vehicle centered in its lane and detect slower vehicles ahead.

Many intersections have some sort of mechanism for detecting vehicles as they approach the intersection.

Inductive loops can be placed in a roadbed to detect vehicles as they pass through the loop's magnetic field.

High-technology systems like infrared sensors along the exit margins, which can detect vehicles moving in the wrong direction, might have helped.

Cross Traffic Alert detects vehicles moving towards the car when the transmission is in reverse.