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Traffic light
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Traffic light

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.

Table of contents
1 Introduction
2 History
3 Pedestrian Scrambles
4 Synchronization
5 Preemption
6 Unusual traffic-light usages
7 External links

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 you 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.

Traffic lights for special vehicles (such as buses or trams) might use other systems, such as vertical vs. horizontal bars of white light.

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.

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].

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.

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 to deal with traffic patterns. Cameras or sensors can be used to monitor traffic patterns across a city. The most high-end systems use dozens of sensors and cost millions of dollars per intersection, but can very finely control traffic levels.

In some areas traffic lights may also be turned off late at night when traffic is very light, sometimes 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, or sometimes the lights are marked as operating at set times only.

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.

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 left-turn signal lights.

In British Columbia, 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.

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.

External links