The traffic light or traffic signal is a device that signals pedestrians and vehicles in alternating use of streets and byways. While signal lights are used in most forms of transportation and pedestrian lamps frequently sit nearby, the contemporary automotive version is most common and iconic. Today they are an arrangement of electric lamps designed to signal multiple directions of traffic using a familiar set of lights: red (stop), yellow/amber (yield or clear the intersection), and green (proceed).
While jaywalking in NYC might appear common, the image of a street cluttered with cars, bicycles and pedestrians in third world cities remains a remarkable symbol of disorder for those unused to it. The difference between the modern order we are taught to expect and dangerous chaos seen in its absence often appears as the orderly flow of traffic. The modern traffic signal often appears as a symbol of civilization or development with the phrase “one traffic light town” synonymous for a place far from the center, or rural. Traffic lights are part of what made the modern orderly city possible and their global use is considered a vital part of proper transportation safety.
The earliest predecessors to modern traffic lights were mechanical signals developed in the nineteenth century to direct trains at track switch points and in rail yards. After their adoption in rail service versions were developed to discipline and organize flows of all sorts of pedestrian and vehicular traffic. The world's first traffic signals, invented by J. P. Knight, were installed near London's House of Commons (intersection of George and Bridge Streets) in 1868. These were the first two-color manual traffic signals utilizing the colors red and green. The color red had already been standard for rail signals and in flag signaling as sign for stop or warning when Knight adopted it. This one lamp was installed for directing horse carts at a busy intersection, and was manned by a police officer. This system was effective but relied on dangerous gas lamps. In January 1869 a police officer operating the device was hurt when the assembly of both gas lamps exploded.
The traffic light is essentially composed of two parts, traffic signals themselves and an external control system. Until the development of electronic and computerized switching in the nineteen fifties traffic lights were switched by a human operator on site or by mechanical extension from a distance. By the time control systems had been automated the traffic light we know had taken form. Traffic lights have been automated as technologies have advanced, but seventy years followed Knight’s pair of lamps in which the control system was based on human action. The history of traffic signals is in many ways a microcosm of lighting technology and energy sources. The earliest signals were entirely human powered hand gestures or signs held by an officer, followed by semi-automated gas lamps in the age of gaslight, and then semi automated electric lamps using incandescent bulbs in the twentieth century. As the technology of incandescent halogen bulbs were developed these took the place of earlier versions, thus providing more light and lowering upkeep maintenance costs. In recent years with concern over energy use spurring research on LEDs (light emitting diodes) or small light bulbs that use a different principle than older bulbs and require less electricity, traffic lights are once again switching throughout the world.
By 1911 Lester Wire of Salt Lake City, Utah had acquired the first patent for an electric traffic signal. Like Knight’s gas lamp in London, Wire’s system was a manually operated pair of red and green lights that would be switched by a police officer. Another early attempt that used electric lamps to direct traffic was patented by James Hoge in 1913; Hoge’s electric lamp system was installed in Cleveland, Ohio and marketed more broadly a year later by the American Traffic Signal Company. Like Wire’s system Hoge’s lamps were reliant on a local operator but Hoge's lights were in the shape of the words "stop" and "move". Either because they were too expensive to be mass produced and or because they were not seen as important, neither system expanded beyond Salt Lake and Cleveland.
Around 1918 a multiple signal device was developed and patented in Cleveland by the prominent African American inventor Garrett Morgan more commonly remembered as the inventor of the first gasmask originally called ‘the Morgan Safety Hood’. Morgan’s device was purchased and produced by General Electric Corporation and became the first mass-produced traffic signals. Morgan’s system used metallic flags mounted on a pole that were switched by a police officer, this system was considered culturally familiar because it was similar to a language of flag signals used by sailors called semaphore and was the first system to be distributed widely. Semaphore signals were used to direct ship traffic in ports and communicate between ships, and while few people today are familiar with semaphore, it was a ready comparison for planners at the time. Though it looked little like contemporary traffic lights, Morgan’s system was the first widely distributed traffic signaling device to manage the rising number of automobiles.
In 1917, at about the same time that Morgan developed his cheap and easy-to- manufacture system, William Ghiglieri of San Francisco, California patented an early form of automatic traffic signal using lights in the- by then traditional- pair of colors red and green. But like Earnest Sirrine of Chicago, Illinois who invented an automatic street traffic signal in 1910, Ghiglieri’s device did not get distributed. Sirrine’s system used the non illuminated words "stop" and "proceed" on mechanical flags and was probably the first system not to involve a human operator. Both systems used mechanical timer systems to automate the changing signal, a precursor of later systems but did not spread at the time.
Sirrine’s system may have been known to William Potts, a police officer in Detroit. Potts is often credited with the invention of the traffic light. In 1920 Potts invented the first four-direction hanging electric three color light, similar to those still in use today. Along with his addition of a yellow light to signal a change, Potts was the first inventor to develop a single unit that faced multiple directions. These two features of the modern lamp can be traced with certainty to his legacy. The history of the traffic light is contentious, containing more than 30 different patents before Potts’ version was introduced in Detroit unpatented. In 1928 a legal battle arose between the manufacturer of Potts’ traffic light and a competitor. Later in that year the United States District Court of Appeals in Chicago decided that Potts should be credited with constructing and initiating the use of the first four-way traffic signal, finding against the competitor. This legal decision names Potts as the inventor of the traffic light despite his not holding patents, and discounts all the other inventors including Morgan.
One of the key developments since the 1930s has been more complicated and responsive systems of automation, starting with centralized control systems of the nineteen fifties that connected or monitored thousands of traffic lights using mechanical switches. The result of this slow development was that the adoption of the traffic lights began slowly. Traffic lights required manual control and were an expensive change over a small flag for a police officer to wave if needed. For example early versions were used in Manhattan, New York, as early as 1920 but the city of New York as a whole was not switched to a traffic light system until 1953, at which time they used a three color light similar to Potts’ original design, with an integrated system of mechanical timers and switches. The lamp post had an individual timer and switch mechanism in a box mounted on the stand body. The switching devices and a system for monitoring them from a central location have been the main changes since that time as control systems have continued to get more sophisticated. For example, some systems use cameras to see the presence of cars, the more common form has been the ‘inductive loop.’ Since 1994 some traffic lights use inductive loops to respond to the presence of the magnetic metals built into cars. The loop consists mainly of an electrified wire coil buried just under the road surface; the loops conductivity changes if there is a large source of metal above them, and this signals the traffic light to change. This system has been popular for its ability to streamline high traffic areas so that the lights only change when traffic is present that needs to cross. These sensors have lead to complaints both for their participation in surveillance and an inability to sense bicyclists which some people say endangers the cyclists.
Gareth A. F. Edel
Aschler, Kate. 2005. The Works: Anatomy of a city. New York: New York: Penguin.
Murphy, Patricia. 2004. Garrett Morgan: Inventor of the Traffic Light and Gas Mask (Famous Inventors) New Jersey: Enslow Elementary Publishers
Sobey, Edward. 2006. A Field Guide to Roadside Technology. Chicago, Illinois: Chicago Review Press Inc