Traffic Signal


A traffic signal, or stoplight as it is also known, controls vehicle traffic passing through the intersection of two or more roadways by giving a visual indication to drivers when to proceed, when to slow, and when to stop. In some cases, traffic signals also indicate to drivers when they may make a turn. These signals may be operated manually or by a simple timer which allows traffic to flow on one roadway for a fixed period of time, and then on the other road-way for another fixed period of time before repeating the cycle. Other signals may be operated by sophisticated electronic controllers that sense the time of day and flow of traffic to continually adjust the sequence of operation of the signals. Traffic engineers use signals to avoid traffic congestion and improve safety for both motorists and pedestrians alike.

The first illuminated traffic signal was installed in London, England, in 1868. It was manually turned and consisted of two gas lamps, one red and one green, with semaphore arms atop a pole. Shortly after its inauguration it blew up while the lamps were being lit and killed a policeman. The first electric traffic signal was installed in Cleveland, Ohio, in 1914. It consisted of a green and red light with a warning buzzer to indicate when the light was about to change. The first signal to use the familiar green, yellow, and red lights was installed in New York City in 1918. It was operated manually from an elevated observation post in the middle of the street. In Los Angeles, traffic lights consisted of green and red lights used in conjunction with a warning gong and a pair of semaphore arms lettered "stop" and "go."

A modern traffic signal system consists of three basic subsystems: the signal lights in their housing, the supporting arms or poles, and the electric controller. The signal lights and housing are known as the signal light stack. A single stack usually consists of three lights: a green light on the bottom to indicate the traffic may proceed, a yellow light in the middle to warn traffic to slow and prepare to stop, and a red light on the top to indicate the traffic must stop. Because some people are red-green color blind, there has been an effort to standardize on a vertical stack of lights with red at the top so that these people can perceive the signal condition by the position of the light rather than the color. Each light has a fresnel lens which may be surrounded or hooded by a visor to make it easier to see the light in bright sunlight. A fresnel lens consists of a series of concentric angled ridges on the outer surface of the lens which bend the light to focus it in a parallel beam. The light stack may have a dark-colored backing plate to make the signals more distinguishable by blocking out surrounding lights from buildings and signs. There are one or more signal light stacks for each direction of each roadway. The electric controller is usually mounted in a weather-proof box on one of the corners of the intersection. More elaborate traffic signals may also have electromagnetic sensors buried in the roadway to detect the flow of traffic at various points.

Raw Materials

The housing or body of each signal light stack is usually made of corrosion-resistant aluminum. Some housings are made of molded polypropylene plastic. The lens for each light is made of tinted glass or plastic. The bulb, known as the lamp, is designed for long life. It is purchased from a light bulb manufacturer. The bulb is partially. surrounded by a polished metal reflector to direct the light forward. The hood or visor is made from aluminum or molded plastic.

The supporting arms or poles are usually made of galvanized steel for strength and corrosion-resistance. They may also be made of fiberglass. The controller is housed in a steel or aluminum enclosure. The electrical components within the controller—switches, relays, and timers—are purchased from various electrical component manufacturers. The wiring between the components is copper with a heavy neoprene rubber or plastic insulation.

The Manufacturing

A traffic signal is fabricated in the manufacturer's plant, then installed and wired at the site of the intersection.

Installed in Detroit in 1920, this was the nation's first three-color, four-way traffic light. (From the collections of Henry Ford Museum & Greenfield Village.)
Installed in Detroit in 1920, this was the nation's first three-color, four-way traffic light.
(From the collections of Henry Ford Museum & Greenfield Village.)

The introduction of the automobile at the turn of the century was chiefly a matter of novelty, and early streets were not paved, engineered, or controlled for automobiles. The first autos joined horse-drawn carriages, pushcarts, horses, bicycles, and pedestrians on the streets.

Initially traffic control was simply non-existent. People and vehicles negotiated their own way through intersections without control. In urban areas police officers eventually tried to direct traffic and soon employed "Stop-Go" hand signs. Problems constantly arose as officers changed the signals with no advanced warning, and motorists found themselves stopping in the middle of the intersection or coming to a near stop each time they approached an intersection. Either way, traffic did not flow so much as it lurched along.

Morgan Garrett, an African American entrepreneur and inventor, experimented with automatic traffic signals in the early 1920s. His first version, placed in the middle of the intersection, worked like a railroad semaphore, with the arms moving to different positions to signal drivers coming from different directions. Garrett's main innovation was the introduction of an intermediate position, the equivalent of yellow on a modern traffic light. This allowed motorists to anticipate a change and slow down only when necessary. Garrett later sold his invention to the General Electric Co., which later made electric, three-light, four-way traffic signals.

William S. Pretzer

Making the signal stack

Making the controller

Making the supports


Quality Control

The manufacturing process for traffic signals is subject to the standard inspections and control practices found in any similar production facility. These include both conventional and statistical methods. The installation on the job site is subject to review by an electrical inspector from the agency placing the signal. Wiring must comply with the National Electrical Code. The location of the light and any other structural considerations must also meet various federal, state, and local ordinances. A registered professional engineer must review and approve the plans to ensure the installation meets the national requirements for traffic control devices.

The Future

With the ever-increasing use of computers, traffic signals in the future will become more sophisticated. Many systems already feature a remote-controlled activation system which allows fire engines and other emergency vehicles to change the signal to green in their direction as they approach an intersection. Some cities are developing networks of traffic signal controllers that interact to keep traffic moving through heavily congested areas or reroute traffic during peak traffic hours. Other advances might include integrating speed warning devices and systems to check for stalled traffic or accidents.

Where To Learn More


Loggins, J.W. "New Traffic Signal Concepts." Public Works. March 1993.

Nadis, S.J. "Gridlock Terminator: Neural Nets Predict the Traffic Future." Omni, October 1994, p. 22.

Douglas E. Betts

Chris Cavette

User Contributions:

If anyone can tell me,I see nowadays PLCs are becoming to be more efficient than anything else when it comes to controling traffic lights, I want to know how the traffic lights were controlled before programmable logic controllers were manufactured.

Thank you

Nowadays am hearing a buzzer at traffic junctions alongwith a green light that appears when pedestrians can cross the road.So what is that buzzer sound for and how is it to be treated,is it a signal for the pedestrians crossing the road or for the riders on vehicle and how exactly it works like

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