People and vehicles negotiated their own way through intersections without control. Initially traffic control was simply non-existent. The first autos joined horse-drawn carriages, pushcarts, horses, bicycles, and pedestrians on the streets. 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 Manufacturing ProcessĪ traffic signal is fabricated in the manufacturer's plant, then installed and wired at the site of the intersection. The wiring between the components is copper with a heavy neoprene rubber or plastic insulation. The electrical components within the controller -switches, relays, and timers -are purchased from various electrical component manufacturers. The controller is housed in a steel or aluminum enclosure. The supporting arms or poles are usually made of galvanized steel for strength and corrosion-resistance. The hood or visor is made from aluminum or molded plastic. surrounded by a polished metal reflector to direct the light forward. It is purchased from a light bulb manufacturer. The bulb, known as the lamp, is designed for long life. The lens for each light is made of tinted glass or plastic. The housing or body of each signal light stack is usually made of corrosion-resistant aluminum. More elaborate traffic signals may also have electromagnetic sensors buried in the roadway to detect the flow of traffic at various points. The electric controller is usually mounted in a weather-proof box on one of the corners of the intersection. There are one or more signal light stacks for each direction of each roadway. 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. 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. 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. 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. 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. The signal lights and housing are known as the signal light stack. 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."Ī modern traffic signal system consists of three basic subsystems: the signal lights in their housing, the supporting arms or poles, and the electric controller. It was operated manually from an elevated observation post in the middle of the street. The first signal to use the familiar green, yellow, and red lights was installed in New York City in 1918. It consisted of a green and red light with a warning buzzer to indicate when the light was about to change. The first electric traffic signal was installed in Cleveland, Ohio, in 1914. Shortly after its inauguration it blew up while the lamps were being lit and killed a policeman. It was manually turned and consisted of two gas lamps, one red and one green, with semaphore arms atop a pole. ![]() The first illuminated traffic signal was installed in London, England, in 1868. Traffic engineers use signals to avoid traffic congestion and improve safety for both motorists and pedestrians alike. 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. 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. ![]() In some cases, traffic signals also indicate to drivers when they may make a turn. 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.
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