These usually include solid red, yellow, and green lights and sometimes red, yellow and green turn arrow lights as well. The Signal Head can contain one or more signal faces. Detectors are devices for indicating the passage or presence of vehicles. In Bakersfield these usually consist of wire loops placed in the pavement at intersections. They are activated by the change of electrical inductance caused by a vehicle passing over or standing over the wire loop.
Some intersections now use a video camera system for detection.
This involves a fixed camera aimed at oncoming traffic. When a vehicle crosses or arrives at a pre-defined location in the field of view of the camera, the vehicle is detected. The transfer of signal control to a special signal operation is called preemption. Railroad Preemption Railroad preemption is initiated when a train passes over advance detectors located on the tracks ahead of the railroad crossing. The purpose of preemption is to clear tracks of traffic stopped on them by traffic signals.