The Controller is the signal's brain. It consists of a computer that controls the selection and timing of traffic movements in accordance with the varying demands of traffic as registered with the controller unit by detectors. Signal Faces are part of a signal head provided for controlling traffic in a single direction and consisting of one or more signal sections. 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 the Las Vegas Valley, they may 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. On newer signal installations, video detection is being used. The transfer of signal control to a special signal operation is called preemption. Railroad preemption is initiated when a train passes over advance detectors located on the tracks ahead of the railroad crossing.