Los semáforos, también conocidos técnicamente como señales de control de tráfico, son dispositivos de señales que se sitúan en intersecciones viales y otros lugares para regular el tráfico, y por ende, el tránsito peatonal. Se instaló el primer semáforo, diseñado por John Peake Knight, en Londres en 1868. El auge de las señales de tráfico está ligado al rápido aumento del tráfico automovilístico, sobre todo en Estados Unidos después de que Henry Ford introdujo el modelo T en 1908 y lo comenzó a producir en masa a partir de 1913. Por primera vez, los coches eran baratos y lo suficientemente confiables para los desplazamientos en masa.

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Traffic lights, also known as traffic signals, traffic lamps, traffic semaphore, signal lights, stop lights, robots (in South Africa), and (in technical parlance) traffic control signals, are signalling devices positioned at road intersections, pedestrian crossings, and other locations to control flows of traffic. The world's first, manually operated gas-lit traffic signal was short lived. Installed in London in December 1868, it exploded less than a month later, injuring or killing its policeman operator. Traffic control started to seem necessary in the late 1890s and Earnest Sirrine from Chicago patented the first automated traffic control system in 1910. It used the words "STOP" and "PROCEED", although neither word lit up. Traffic lights alternate the right of way accorded to users by displaying lights of a standard color (red, amber (yellow), and green) following a universal color code. In the typical sequence of color phases: \n* The green light allows traffic to proceed in the direction denoted, if it is safe to do so and there is room on the other side of the intersection. \n* The amber (yellow) light warns that the signal is about to change to red. In a number of countries – among them the United Kingdom – a phase during which red and yellow are displayed together indicates that the signal is about to change to green. Actions required by drivers on a yellow light vary, with some jurisdictions requiring drivers to stop if it is safe to do so, and others allowing drivers to go through the intersection if safe to do so. \n* A flashing amber indication is a warning signal. In the United Kingdom, a flashing amber light is used only at pelican crossings, in place of the combined red–amber signal, and indicates that drivers may pass if no pedestrians are on the crossing. \n* The red signal prohibits any traffic from proceeding. \n* A flashing red indication is treated as a stop sign. In some countries traffic signals will go into a flashing mode if the controller detects a problem, such as a program that tries to display green lights to conflicting traffic. The signal may display flashing yellow to the main road and flashing red to the side road, or flashing red in all directions. Flashing operation can also be used during times of day when traffic is light, such as late at night.

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