There are always two main lights, a red one which means stop, and a green one which means go. In some countries there is also a yellow (or amber) light. If the amber light is switched on and unflashing you should stop if you are safely able to do so. In some systems, a flashing amber means that you may go ahead with care if the road is clear, giving way to pedestrians, and to other road vehicles that may have priority. There may be additional lights (usually a green arrow or "filter") to authorize turns.
In most countries, the sequence is red (stop), green (go), amber (prepare to stop). In the UK, the sequence includes red + amber together before green, which helps draw attention to the impending change to green, to allow drivers to prepare to move off. The single flashing amber signal is only used in the UK at Pelican crossings.
Depending on the jurisdiction, traffic may turn after stopping on a red (right in right driving countries; left in left driving countries).
On December 10, 1868, the first traffic lights were installed outside the Houses of Parliament in London. They resembled railway signals of the time, with semaphore arms and red and green gas lamps for night use.
The modern electric traffic light is an American invention. As early as 1912, Salt Lake City policeman Lester Wire set up the first red-green electric traffic lights. On Aug. 5, 1914, the American Traffic Signal Company installed a traffic signal system on the corner of 105th Street and Euclid Avenue in Cleveland. Based on the design of James Hoge, it had two colors, red and green, and a buzzer to provide a warning for color changes. The first three-color traffic lights were introduced in New York and Detroit in 1920.
Traffic lights for pedestrians are usually different, see pedestrian crossing.
A pedestrian scramble is a special traffic light that stops all vehicular traffic. Pedestrians then have exclusive access to the intersection and can cross diagonally across the intersection. Pedestrian scrambles are useful when there is heavy diagonal pedestrian traffic or heavy pedestrian traffic in general. In intersections with heavy pedestrian traffic, pedestrians have the right of way blocking drivers from turning. A pedestrian scramble gives vehicles exclusive access to the intersection for a period of time as well.
Traffic lights at level railroad crossings are again different.
Traffic lights are sometimes centrally controlled to allow them to be coordinated and timed such that traffic in certain directions can catch all green lights. This timing may be altered during different parts of the day depending on traffic conditions. Traffic lights may also be turned off late at night when traffic is very light: traffic in the main street gets a flashing amber to warn of the intersection; traffic in the secondary street gets a flashing red to indicate a stop before proceeding into the intersection.
Traffic light failure in most jurisdictions is to be handled by drivers as a 4-way stop sign pending the arrival of a police officer to direct traffic.
Irregular uses of normal lighting systems
In parts of Canada (the Maritime Provinces, Ontario and Alberta), a flashing green light has a special meaning. It is similar to the left turn signal that is attached to a standard green. (Either in the "dogleg" pattern or attached to the bottom of the standard signal. Not the standalone.) In Ontario, this usage is slowly being phased out in favour of left-turn signal lights.
In British Columbia, a flashing green signal is used to signify a crosswalk, when the pedestrian has the ability to stop traffic to allow a safe crossing.
In parts of Mexico, the green lights will start flashing at the end of the Go or Turn phase to indicate that the amber (Caution phase) lights are about to be engaged.
Other places where there may be traffic lights (normal or special ones):