<P> The control of traffic lights made a big turn with the rise of computers in America in the 1950s . Thanks to computers, the changing of lights made Crosby's flow even quicker thanks to computerized detection . A pressure plate was placed at intersections so once a car was on the plate computers would know that a car was waiting at the red light . Some of this detection included knowing the number of waiting cars against the red light and the length of time waited by the first vehicle at the red . One of the best historical examples of computerized control of lights was in Denver in 1952 . One computer took control of 120 lights with six pressure - sensitive detectors measuring inbound and outbound traffic . The system was in place at the central business district, where the most traffic was between the downtown area and the north and northeastern parts of the city . The control room that housed the computer in charge of the system was in the basement of the City and County Building . As computers started to evolve, traffic light control also improved and became easier . In 1967, the city of Toronto was the first to use more advanced computers that were better at vehicle detection . Thanks to the new and better computers traffic flow moved even quicker than with the use of the tower . The computers maintained control over 159 signals in the cities through telephone lines . People praised the computers for their detection abilities . Thanks to detection computers could change the length of the green light based on the volume of waiting cars . The rise of computers is the model of traffic control which is now used in the 21st century . </P> <P> Countdown timers on traffic lights were introduced in the 1990s . Timers are useful for pedestrians, to plan whether there is enough time to cross the intersection before the end of the walk phase, and for drivers, to know the amount of time before the light turns green . In the United States, timers for vehicle traffic are prohibited, but pedestrian timers are now required on new or upgraded signals on wider roadways . </P> <P> Aspect is a term referring to the light that faces the drivers . A typical vertical traffic signal has three aspects, or lights, facing the oncoming traffic, red on top, yellow below, and green below that . Generally one aspect is illuminated at a time . In some cases, a fourth aspect, for a turn arrow for example, is below the three lights or aspects in more complicated road traffic intersections . </P> <Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (March 2010) </Td> </Tr> </Table>

Where is the green light on a traffic light