<P> In some countries traffic signals will go into a flashing mode if the conflict monitor detects a problem, such as a fault 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 . </P> <P> Before traffic lights, traffic police controlled the flow of traffic . A well - documented example is that on London Bridge in 1722 . Three men were given the task of directing traffic coming in and out of either London or Southwark . Each officer would help direct traffic coming out of Southwark into London and he made sure all traffic stayed on the west end of the bridge . A second officer would direct traffic on the east end of the bridge to control the flow of people leaving London and going into Southwark . </P> <P> On 9 December 1868, the first non-electric gas - lit traffic lights were installed outside the Houses of Parliament in London to control the traffic in Bridge Street, Great George Street, and Parliament Street . They were proposed by the railway engineer J.P. Knight of Nottingham who had adapted this idea from his design of railway signalling systems and constructed by the railway signal engineers of Saxby & Farmer . The main reason for the traffic light was that there was an overflow of horse - drawn traffic over Westminster Bridge which forced thousands of pedestrians to walk next to the Houses of Parliament . The design combined three semaphore arms with red and green gas lamps for night - time use, on a pillar, operated by a police constable . The gas lantern was manually turned by a traffic police officer with a lever at its base so that the appropriate light faced traffic . The signal was 22 feet (6.7 m) high . The light was called the semaphore and had arms that would extend horizontally that commanded drivers to "Stop" and then the arms would lower to a 45 degrees angle to tell drivers to proceed with "Caution". At night a red light would command "Stop" and a green light would mean use "Caution". </P> <P> Although it was said to be successful at controlling traffic, its operational life was brief . It exploded on 2 January 1869 as a result of a leak in one of the gas lines underneath the pavement and injured the policeman who was operating it . </P>

When were traffic lights first used in the uk