When installed under conditions that justify its use, a traffic signal is a valuable device for traffic control. However, an ill-advised or poorly designed signal is not only annoying, but can be dangerous to pedestrians, cyclists and drivers. Therefore, it is essential that, before traffic signals are installed, engineering studies be made by qualified personnel.

A traffic signal provides alternate right-of-way for different traffic movements at an intersection. It provides a degree of control that is second only to physical barriers. A good general guide is to use the least traffic control required to provide for the safe and efficient movement of vehicles and pedestrians. Specifications for signals and

their placement as well as warrants for their use are contained in a publication entitled, "Manual on Uniform Traffic Control Devices" published by the U.S. Department of Transportation.

A signal that minimizes vehicle stops and delay also cuts fuel consumption and emissions. The signal controller switches the signal indications on and off to assign right-of-way correctly and safely.

Two basic kinds of controllers are used: Pretimed (also known as fixed-time) and traffic-actuated.

Pretimed controllers operate on a predetermined, regularly repeated sequence of signal indications. They are used frequently where traffic volumes are predictable and stable.

Traffic-actuated controllers differ from pretimed controllers in that their signal indications are not of fixed length, but change in response to variations in traffic demand. They are frequently used where traffic volumes fluctuate widely or irregularly, or where interruptions to major street flow must be minimized.

Signal Timing is the division of the cycle into seconds for each of the phases. It assigns right-of-way to alternate traffic movements in order to reduce traffic delay and accident-producing conflicts. Signal timing is constrained by the cycle length - the time for one complete sequence of the signal indications. Cycle lengths usually fall between 45 and 120 seconds.

There are three common techniques for coordinating traffic signals to operate as a system. This is done to improve the progressive flow of traffic along an arterial street or in a network, any of which can work with either pretimed or actuated controllers.

The simplest system is the basic programmed system in which a master controller simply sends a periodic pulse to all intersections to instruct the local controllers that this is the system reference point.

The second method, called "time based coordinators", replaces the central controller and the interconnecting communications completely and places a very accurate timer directly at each location.

The third system uses a sophisticated central computer control that exerts more external control on the individual controllers.