Picture this situation – you are waiting at a red traffic signal. The seconds are passing, and you are wondering why is the signal not turning green? At that point, a thought is passing through your mind – who invented these things after all? Why did it invented it to work in this way? And why is the signal having three colors? Well, for all those of you who ever had similar thoughts, the following historical review might give you some answers.
We need to start this historical retrospective some century and a half ago. In 19th century, more precisely in 1868, railway engineer John Peake Right [1, 2] invented the first illuminated traffic signal. Considering that signal lights were deployed at railroads as early as 1857 , this might be the origin of Right’s idea. The first traffic signal was installed on December 10, 1868, outside of the British Houses of Parliament in London, at the intersection of George Street and Bridge Street [3, 4]. This manually operated traffic light  had green and red lenses illuminated by gas for night operation . The signal was primarily used for enabling pedestrian crossing . The official proclamation accompanying this device , issued by the London Police Commissioner, was “By the signal “caution”, all the persons in charge of vehicles and horses are warned to pass over the crossing with care and due regard to the safety of the font passengers. The signal “stop” will only be displayed when it is necessary that vehicles and horses shall be actually stopped on each side of the crossing, to allow the passage of persons on foot; notice thus being given to all persons in charge of vehicles and horses to stop clear of the crossing”. This is how the original engineering solution for the issue of intersection flows control was established on the principle of separating conflicting flows of users. The improved, safer for operation, design of this signal was installed in 1872 .
Although Europe was the place of the first traffic signal, due to the increase in urban population and number of motor vehicles, the early developments were primarily happening in the US. Starting from 1909, there were first patents for traffic control devices in the US. In 1912 Salt Lake City, a police detective Lester Wire  designed a two-color traffic signal that used electric illumination. Two years later, in 1914, the American Traffic Signal Company installed the first electric two-color signals in Cleveland, Ohio [6, 10] on the corner of 105th Street and Euclid Avenue . The traffic signal faced two main street directions, with police officers controlling side street traffic and manually changing the lights.
In 1917, William Caglieri was the first to come up with the idea that signals should automatically change colors at set intervals. The design involved electrically illuminated red and green lenses. The timing mechanism operated as a clock, using a weight suspended from a cord that was rolled onto a drum, and had to be wound on a regular basis. Patent records show that this was the first use of the term “traffic signal.” In 1918, Charles Reading helped Lester Wire to link six traffic lights in Salt Lake City, Utah, to provide coordination for traffic traveling at 18 mph (30 km/h) [7, 12].In 1918, in Detroit, police superintendent William Potts added a third yellow color to the signal, as a caution interval, warning drivers of the changing signal . In 1920, Mr. Potts [3, 13] designed the first four-way three-color traffic control device. The device was a rectangular box, having three stacked chambers with each chamber having a single light bulb. Because a single light bulb lit the four lenses in each compartment of this signal, the green and red lenses were alternating between top and bottom positions (Figure 2). This design completed the foundations of the modern traffic signals, and was the basis for the following decades of fixed time traffic signal control. After the early development and implementations in the US, Western Europe widely adopted traffic signals several years later. Paris installed its first red and two-way only light in 1922, with Berlin and London following in 1924 and 1931, respectively . The US maintained to lead the development of traffic signal equipment. Later devices were characterized by their individualistic design. The market contained a wide variety of traffic regulating devices, ranging from improvements to simple semaphore devices to tall apparatuses that covered entire intersections. Most of these devices relied on cables and pulleys or underground crank mechanisms used by traffic police from the corners of intersections. Only few of these devices had automatic capabilities. In 1923, a railway signal company Crouse-Hinds stationed in Houston, developed the first automatic variable timing switch using induction disc that could vary the cycle length and split [3, 11]. In the late 1920s and early 1930s, a significant change arrived with the interest of big engineering companies into traffic signal equipment. These companies with large engineering teams established a niche for a serious market manufacturing signals and traffic signal controllers . One of the examples was when General Electric bought the patent from G. A. Morgan, one of the inventors of traffic regulating devices. By the 1940s, there were several companies manufacturing traffic signal equipment. Figure 3 is an example of brochure by Eagle Signal Corp. presenting different controller devices manufactured.
In order to further eliminate manual labor in traffic signal control, some inventors worked on developing means for detecting vehicles. Charles Adler, Jr.  was the first to invent the acoustic detector operation for a traffic signal. His design was based on the microphone mounted on a pole at an intersection, that could detect a vehicle’s horn honk or engine rumbling. Sonic vibrations would activate the mechanism that shifts the electrical circuits and change the light. In 1928 a horn-actuated signal was installed near Baltimore, Maryland . About the same time, Henry A. Haugh developed a pressure detector. The detector was designed to send electrical impulses to the signal controller when two metal strips touch under vehicles pressure. By 1934, loop detectors, as an emerging technology started to be widely used , and replaced most of the other detector technology.
Looking back through this brief historical retrospective on the origins of traffic signals and related technology, it is evident that underlying technological concepts are still quite similar. The principle of conveying the control message to traffic users through the change of signal indication that Mr. Right designed long time ago is present in the conventional signal control systems. The new developments in computer and communication technology could enable this perspective change, giving a unique opportunity to reinvent the basic premises of traffic signal control systems. However, with a thorough understanding of the foundations of conventional control and its objectives, that future technology has a change to develop from innovation, and not vice versa.
 T. Urbanik, D. Bullock, L. Head, D. Gettman, R. Campbell, M. Abblett, E. Smaglik, S. Beaird, J. Yohe, and S. Quayle, “NCHRP 3-66: Traffic Signal State Transition Logic Using Enhanced Sensor Information,” National Cooperative Highway Research Program – Transportation Research Board 2006.
 Traffic Signal Maintenance Handbook: Institute of Transportation Engineers, International Municipal Signal Association 2010.
 E. A. Mueller, “Aspects of the history of traffic signals,” Vehicular Technology, IEEE Transactions on, vol. 19, pp. 6-17, 1970.
 “National Traffic Signal Report Card – Technical Report,” National Transportation Operations Coalition2005.
 P. Koonce, L. Rodegerdts, K. Lee, S. Quayle, S. Beaird, C. Braud, J. Bonneson, P. Tarnoff, and T. Urbanik, “Traffic signal timing manual,” 2008.
 P. Yauch, “Traffic Signalization-A History,” 1997.
 M. G. Lay and J. E. Vance, Ways of the World: A History of the World’s Roads and of the Vehicles that Used Them: Rutgers University Press, 1999.
 “A History of Traffic Control Devices,” Institute of Transportation Engineers, Washington, D.C.1980.
 D. Miller, “Advanced Transportation Controller Standards and Interoperability,” ITE Journal, 2010.
 “NEMA Standards Publication No. TS1 – 1983,” National Electrical Manufacturers Association, Washington, D.C. 1983.
 C. McShane, “The origins and globalization of traffic control signals,” Journal of Urban History, vol. 25, pp. 379-404, 1999.
 D. C. Gazis, Traffic theory vol. 50: Kluwer Academic Publishers, 2002.
 Traffic Signal Field Technician – Level II International Municipal Signal Association 2009.
 (03/28/2012). Willis Lamm’s Traffic Signal Collection – History of Traffic Signal Design. Available: http://www.kbrhorse.net/signals/history01.html
 “2012 National Traffic Signal Report Card – Technical Report,” National Transportation Operations Coalition2012.