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Traffic signal)
Traffic lights will sometimes differ where there are several lanes of traffic. In this example, the lowest light is a filter for right turns.
A traffic light or traffic signal is a device positioned at road intersections or pedestrian crossings to indicate when it is safe to drive, ride or walk, using a universal color code.
Introduction
Traffic lights for normal vehicles or pedestrians always have two main lights, a red one which means stop, and a green one which means go. In most 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 a motorist may go ahead with care if the road is clear, giving way to pedestrians, and to other road vehicles that may have priority. A flashing red essentially means the same as a regular stop sign. There may be additional lights (usually a green arrow or "filter") to authorize turns (called a lead light in the U.S., because it is usually leading the main green light).
Traffic lights for special vehicles (such as buses or trams) might use other systems, such as vertical vs. horizontal bars of white light.
4-state lights warn traffic that it will shortly be free to move.
In most countries, the sequence is red (stop), green (go), amber (prepare to stop). In the UK, Germany and Poland, among others, 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 used in the UK and Australia 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). In some jurisdictions which generally forbid this, a green arrow sign next to the traffic light indicates that it is allowed at a particular intersection. Conversely, jurisdictions which generally allow this might forbid it at a particular intersection with a "no turn on red" sign. In France permissission to turn right on a red light is indicated by a smaller set of two lights by the main ones that indicate green to permit this action or red to forbid it and these will be set change if there is an ajoining pedestrian crossing signal.
Traffic light failure in most jurisdictions must be handled by drivers as a four-way stop, pending the arrival of a police officer to direct traffic. Some jurisdictions (e.g. Switzerland), however, have additional rights of way signs mounted above the traffic lights; these kick into effect when the lights are no longer active.
History
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 August 5, 1914, the American Traffic Signal Company installed a traffic signal system on the corner of 105th Street and Euclid Avenue in Cleveland, Ohio. 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.
The first automatic traffic lights could be seen in Wolverhampton, England in 1927.
For unknown reasons, Garrett Morgan is sometimes mistakenly credited as the inventor of the traffic light. See [1] (http://www33.brinkster.com/iiiii/trfclt/).
Pedestrian scrambles
A pedestrian scramble, or Barnes Dance (named for Henry Barnes), 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.
Hachiko Square, in Shibuya, Tokyo, has a famous pedestrian scramble at an intersection of seven streets (some pedestrian-only) in front of Shibuya Station.
In the United States, the city of Beverly Hills is famous for being the first California city to implement diagonal crossing (at some intersections on Rodeo Drive).
Synchronization
Attempts are often made to synchronize traffic lights so that drivers encounter long strings of green lights. This is only easily done on one-way streets with fairly constant levels of traffic. Two-way streets are often arranged to correspond with rush hours to speed the heavier volume direction. Congestion can often throw off any synchronization, however.
More recently even more sophisticated methods have been employed. Traffic lights are sometimes centrally controlled by monitors, or by computers, to allow them to be coordinated in real time to deal with changing traffic patterns. Video cameras, or sensors buried in the pavement can be used to monitor traffic patterns across a city. Unsynchronized sensors occasionally impede traffic, by detecting a lull and turning red just as cars arrive from the previous light. To prevent this, the most high-end systems use dozens of sensors and cost millions of dollars per intersection, but can very finely control traffic levels. This relieves the need for other measures (like new roads) which are even far more expensive.
In some areas traffic lights may also be turned off late at night when traffic is very light. Under these circumstances, traffic in the main street may get a flashing amber to warn of an intersection. Traffic in the secondary street gets a flashing red (see above), or sometimes the lights are marked as operating at set times only. Some lights outside of fire stations have no green, as they may only turn amber for a few seconds and red for a few minutes a week.
Preemption
Some regions have signals that are interruptible, giving priority to special traffic. This is usually reserved for emergency vehicles such as ambulances and police squad cars, though sometimes mass transit vehicles including buses and light rail trains can interrupt lights. There have been some concerns that unauthorized people may have obtained devices that can trigger light preemption. Most of the systems operate with small transmitters that send radio waves or infrared signals that are received by other devices on or near the traffic lights. Sometimes, an additional signal light is placed nearby to warn motorists that an oncoming vehicle is preempting the signals.
Unusual traffic-light usages
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 the more universally-understood left-turn signal lights.
In British Columbia and in Massachusetts, a flashing green signal is used to warn of a crosswalk at which pedestrians have the ability to stop traffic to allow a safe crossing.
In Austria, parts of Mexico, Turkey, and Russia, 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. This is useful in fast paced roads to allow for longer slowing down time.
In Kraków (and probably some other parts of Poland) there are signs displaying how fast one has to drive in order to reach the next intersection at the exact time when the light turns green. It is very useful in heavy traffic, but also very uncommon.
In some parts of the Northeast USA and Georgia, traffic lights have been fitted to slowly strobe white lights superimposed on the center of the red light when the red light itself has been illuminated. These seem to be located in situations where the driver may have been travelling for a length time without seeing any traffic lights (such as a controlled-access highway) or in other situations where extra work may be needed to draw attention to the status of the light.
Tianjin in the People's Republic of China has two very special systems of traffic lights:
- One system is where there is a horizontal bar in a specific colour, with the colour changing and the bar shrinking. The shrinking bar indicates the time remaining in that colour. The colour itself is either red (stop), yellow or green (go). A blinking green half-bar means "reduce speed now", and a blinking yellow full-bar indicates "proceed with caution".
- Another system is where there is a set of three lights as traffic lights, but every light is an arrow pointing in different directions and every arrow has a colour of its own, to show whether traffic flow is permitted or prohibited in that direction.
Elsewhere in China, a blinking green light means "reduce speed now", attempting to stop cars from passing (if that car can still safely stop in time) and is nearly universal in appearance. Some cities or parts of cities show the number of seconds remaining in a specific traffic light colour (a so-called "countdown meter").
Other places where there may be traffic lights (normal or special ones):
Traffic lights for pedestrians are usually different, see pedestrian crossing. Traffic lights at level railroad crossings are again different. Both of these are to avoid confusion as to whom the signal applies.
External links
de:Lichtzeichenanlage
es:Semįforo
nl:Verkeerslicht
fr:Carrefour ą feux
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