Traffic light

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Traffic lights can have several additional lights for filter turns or bus lanes. This one shows the distinctive red + amber combination seen in the UK.
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A typical American left-turn traffic light.

A traffic light or traffic signal is a signalling device positioned at a road intersection or pedestrian crossing to indicate when it is safe to drive, ride or walk, using a universal color code.



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. Usually, the red light contains some orange in its hue, and the green light contains some blue, to provide some support for people with red-green color blindness. In most countries there is also a yellow (or amber) light, which when on and not flashing means stop if able to do so safely. 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.

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3-state traffic light.
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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. Some older signals in New England, U.S, mainly near Boston, use the red + amber phase to signify an all-pedestrian phase ("Barnes dance"), as they were installed before pedestrian signals came to the area, and before the national standard prohibited them. See "#Pedestrian scrambles" below, or the main article.

Depending on the jurisdiction, traffic may turn after stopping on a red (right in right-driving countries; left in left-driving countries), provided they yield to pedestrians and other vehicles. 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, or might put a green arrow to indicate specifically when a right turn is allowed without having to yield to pedestrians (this is usually when traffic from the perpendicular street is making a left turn onto one's street and thus no pedestrians are allowed in the intersection anyway). Some jurisdictions allow turning on red in the opposite direction (left in right-driving countries; right in left-driving countries) from a one-way road onto another one-way road; some of these even allow these turns from a two-way road onto a one-way road. Also differing is whether a red arrow prohibits turns; some jurisdictions require a "no turn on red" sign in these cases. A study in the State of Illinois concluded that allowing drivers to proceed straight on red after stopping, at specially posted T-intersections where the intersecting road only went left, was dangerous.

In France and Spain permission to turn right (or more rarely to turn left or to go straight on) on a red light is indicated by a flashing amber arrow (cars do not have to stop but must yield way to other cars and pedestrians).

Another distinction is between intersections that have dedicated signals for turning across the flow of opposing traffic and those that do not. Such signals are called dedicated left-turn lights in the United States (since opposing traffic is on the left); it is a "protected" signal if a red arrow appears after the phase; a "permissive" signal, in a configuration known as a "doghouse", has no red left arrow, and the red ball is in the middle, above the left and straight columns. If there is such a signal, it turns green when traffic may turn left without conflict, and turns red or disappears otherwise. If there is no such signal, one must yield to opposing traffic and turn when it is safe to do so. In the U.S., many inner-city and rural areas do not have such dedicated lights, while most suburban areas have them. Such lights tend to make intersections safer and may speed up through traffic by reducing the risk of head-on collisions, but may decrease the overall efficiency of the intersection as it becomes congested, depending on what proportion of traffic is turning.

Intersections without dedicated protected signals occasionally have what is known as a "left turn trap" (in right-driving countries). This happens when traffic proceeding the other way gets a green light for a longer period of time than the direction of a turning vehicle, to allow opposing left turns to proceed (and the other direction may have a "delayed green", allowing left turns in this direction). A driver that has entered the intersection on green to turn left is trapped when the light turns red, as the other direction still has a green light, and the driver has no way of knowing when that direction will change, despite the right to proceed.

Traffic light failure in most jurisdictions must be handled by drivers as a four-way stop (or, in Europe, a priority-to-the-right intersection), pending the arrival of a police officer to direct traffic. Some jurisdictions (e.g. Switzerland, France or Austria), however, have additional right-of-way signs mounted above the traffic lights; these kick into effect when the lights are no longer active. In preparation for Y2K, some jurisdictions installed emergency unfoldable stop signs at intersections.


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An typical example of how traffic lights are mounted in California.
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Wealthy cities like Cerritos often have elaborate traffic light gantries.

There are significant differences from place to place in how traffic lights are mounted or positioned so that they are visible to drivers. Depending upon the location, traffic lights may be mounted on poles situated on street corners, hung from wires strung over the roadway, or even hung from horizontal poles or installed within large horizontal gantries that extend out from the corner and over the right-of-way. In the last case, such poles or gantries often have a lit sign with the name of the cross-street.

Some places mount lights with their multiple faces arranged horizontally and others vertically.

California is particularly fastidious in ensuring that drivers can see the current state of a traffic light. One entrance to a typical large intersection, with three through lanes, two dedicated left-turn lanes, and a crosswalk, may have as many as three traffic lights for the left-turn lanes, three for the through lanes, and a pedestrian signal for the crosswalk. And those numbers must be multipled by four to cover all four ways to enter a typical intersection.

In addition to being positioned and mounted for desired visibility for their respective traffic, some traffic lights are also aimed, louvered, or shaded to minimize mis-interpretation from other lanes. For example, a Fresnel lens on an adjacent through-lane signal may be aimed to prevent left-turning traffic from anticipating its own green arrow.

Shades and backpanels are also useful in areas where sunlight would diminish the contrast and visibility of a signal face.


In the , after continued destruction of a standard traffic light in its   neighborhood, the City of  in the  gave up and installed a traffic light with green on the top. The Irish objected to the fact that "" red was placed above "Irish" green.
In the 1920s, after continued destruction of a standard traffic light in its Tipperary Hill Irish neighborhood, the City of Syracuse in the United States gave up and installed a traffic light with green on the top. The Irish objected to the fact that "British" red was placed above "Irish" green.

On 10 December 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 5 August 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 interconnected traffic signal system could be seen in Salt Lake City, Utah in 1917, with six connected intersections controlled simultaneously from a manual switch. Automatic control of interconnected traffic lights was introduced March 1922 in Houston, Texas.[1] (

The first Automatic experimental traffic lights in England were deployed in Wolverhampton in 1927. [2] (

Garrett Morgan is sometimes mistakenly credited as the inventor of the traffic light. See [3] (

The traffic light is an example of a finite state machine.

Pedestrian traffic lights in East Berlin

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An East Berliner pedestrian traffic light. These have come to be the symbol of East Berlin

The German Democratic Republic had pedestrian traffic lights with a design quite different from the one in use in the West. East Germans strongly resisted the idea of converting to the less interesting West German stick figures during the reunification of Germany, and as a consequence, the little man (the "Ampelmnnchen") of the East Berlin traffic lights has become a symbol of East Berlin. They are so popular that they are features on lots of tourists items and souvenirs.

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Tourist souvenirs featuring the East Berliner traffic lights
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Tourist souvenirs featuring the East Berliner traffic lights

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 diagonally cross 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.

Usually these are displayed as simply a red signal in all directions with walk signals; some older intersections, at least in the Boston area, show both red and amber signals in all directions for this.

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).

In Trondheim, Norway, nearly all the traffic lights in the centre of the city are pedestrian scrambles.


Attempts are often made to synchronize traffic lights so that drivers encounter long strings of green lights. This is done easily only 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.
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This traffic light in Khobar is video camera-synchronized (just above the vertical light) and also shows the seconds remaining to change to the next state (in the horizontal light)

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 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. In many parts of Europe, traffic light-controlled intersections also have yield and right-of way signs incase the signals fail or are turned off. Some lights outside of fire stations have no green, as they may only turn amber and then red while fire trucks are exiting the station en route to an emergency. See also the "Unusual traffic-light usages" described below.


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. In one recent Oregon incident (2005) a fire engine pre-empted a signal and proceeded to collide with a crossing train that had insufficient warning to stop.

Another type of preemption is railroad preemption. Traffic-signal-controlled intersections next to railroad crossings on one of the roads usually have this feature. Approaching trains activate a routine where, before the train signals and gates are activated, all traffic signal phases go to red, except for the signal immediately after the train crossing, which turns green to allow traffic on the tracks to clear (in some cases, there are auxillary traffic signals prior to the railroad crossing which will turn red, keeping new traffic from crossing the tracks). After enough time to clear the crossing, the signal will turn red and the crossing lights will begin flashing and the gates will lower. The operation of a traffic signal while a train is present may differ from municipality to municipality. In some areas, all directions will flash red, turning the intersection into an all-way stop. In other areas, the traffic parallel to the railroad track will have a green light for the duration of the train while the other directions face a red light for the duration of the train. The Chicago Drive/Ivanrest Avenue intersection in Grandville, Michigan, gives Chicago Drive traffic (parallel to the tracks) a flashing yellow with fiber-optic lit signs indicating "no right turn" or "no left turn" over the tracks, and Ivanrest traffic faces a solid red light. In Goshen, Indiana, the signals at the intersections on Linconlway will run normally, with the exception that oncoming traffic (across from the railroad crossing) will face "doghouse" signals with left and right arrows lit: all traffic is required to turn left or right if a train is present, to keep traffic moving.

There has been recent concerns about the security of traffic light preemptive systems and the actual underlying network controlling them and traffic lights in general. An article ( in the hacker E-zine Phrack has outlined flaws in the traffic controlling system that could allow an unauthorized malicious person to abuse it as he sees fit. By issuing valid signal controlling messages from the area traffic control center if gained access to, an attacker could essentially control any phase, test phase, preemptive signals, or any function of the traffic system that is controllable remotely. The article also sparked a response by Transport for London where it is reported that a skilled attacker armed with this step-by-step guide could in fact cause malicious damage, as reported by a Transport for London spokesman.

Unusual traffic-light usages

In parts of Canada (the Maritime Provinces, Quebec, Ontario and Alberta), a flashing green light has a special meaning. It is identical in meaning to signals where one side has both green and a green left-turn arrow, and the opposite side has red (cross-traffic has red as well). The light phase is known as "advanced green", and a sign saying "Advanced green when flashing" is usually attached to the light in question. The opposite side often has a sign attached to their lights saying "Delayed Green Wait for signal." Advanced green indicates that the opposing traffic is facing a red light, and it is safe for you to turn left. In Ontario, older lights with this system are slowly being phased out in favour of more universally-understood left-turn arrow signals.

In British Columbia and Massachusetts and a few other states, 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. They may also be used at a drawbridge. The flashing green indicates that the signal is not currently in use. It changes to solid green for a short time before entering the normal yellow/red/green sequence, then returns to flashing green until another crossing is requested.

In Austria, Estonia, Latvia, most of Israel, 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 Michigan, a flashing red ball signal on a "protected" left turn traffic signal indicates that left turning traffic may, after a full stop, complete their turn if and only if there is a long enough break in oncoming traffic. The flashing red usually occurs when the oncoming traffic has a green signal. This function is not enabled at intersections where it may not be safe to do so (restricted view of oncoming traffic, heavy pedestrian crossings, or double-lane left turns are good examples). It is also worthy to note that Michigan usually indicates signals that are dedicated to turning traffic with a sign displaying "LEFT" or "RIGHT". This sign is normally illuminated at night.

In some cities (such as Kyiv, Ukraine or Krakw, 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, and is typically set to the speed limit (eg. 60km/h).

In some parts of the United States, 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), in a place where a regular traveler wouldn't expect a signal (such as a newly erected signal or one put up for contstruction) or in other situations where extra work may be needed to draw attention to the status of the light (such as in an area where many other red lights approximate the brightness, placement and color of a red traffic signal). These are also used in areas prone to fog, as the strobing white light may be visible from a distance while the standard red light is not.

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The "bar traffic light" in Tianjin (variant one)
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The "multiple arrow traffic light" in Tianjin (variant two)

Tianjin in the People's Republic of China has two very special systems of traffic lights, in use since c. 1999/2000:

  • 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 one-third-full bar means "reduce speed now", and a blinking yellow full-bar indicates "proceed with caution".

When lights of this system turn from green to red, the diminishing green bar will flash once two-thirds (note: not the full bar) of the green bar is "eaten up", with the remaining third intact. A full, uninterrupted yellow bar will appear for a few seconds before, after a short blink, lights turn red. Immediately after the full red bar appears, a tiny (almost unnoticeable) split/division appears to signify the bit that will not be "eaten up". This corresponds to the usual position of a red light (leftmost, or rightmost if at the other end of the road and at the other side of the pavement; or the upper third). When two-thirds of the red bit is "eaten up", the red light extinguishes, only to be replaced nearly immediately with a full chunk of green (again with the minute division). The process then repeats itself.

  • 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.

The major disadvantage of this system of traffic light is that it is unfamiliar to those who are used to seeing specific colours of the traffic lights at the various ends of a normal traffic light itself (e.g. green rightmost, red leftmost, etc)... It does, however, conserve space.

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").

The City of Diadema (Metropolitan Area of So Paulo, Brazil) has special traffic lights that show time remaining (in seconds) for both drivers and pedestrians. At every moment, one can tell how much time one has to pass the crossroads.

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.

On some large toll bridges, such as the San Francisco-Oakland Bay Bridge, red/green traffic lights are used to stagger traffic leading into the bridge. In the Bay Bridge's case, approximately 25 lanes of toll booth traffic are reduced to five lanes of bridge traffic in about 1/2 mile. To accomplish this, an overhead red/green traffic light is visible above each lane, several hundred feet beyond the toll plaza. Green is illuminated for 2 seconds, signalling the first driver in that lane to begin acceleration. Then the signal jumps to red for eight seconds. Using this method, there are always five lanes with a "green" signal, staggered throughout the 25 lanes of traffic.

In areas where there is a high-speed road (at least 55 mph) leading up to a traffic light, where the traffic light is obscured from a distance (or both conditions), or before the first traffic signal after a long stretch of road with no signals, a "prepare to stop" sign with two alternately-flashing amber lights is installed where drivers can view it from a distance. This light begins blinking with enough time for the driver to see it and slow down before the intersection light turns yellow, then red. The flashing amber light can go out immediately when the light turns green, or it may continue for several seconds after the intersection light has turned green, as it usually takes a line of cars some time to accelerate to cruising speed from a red light.

Lane control

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Lane control signals. Flashing red is sometimes used instead of yellow.

On some high-traffic roads which do not have an even number of total lanes, or on bridges or in tunnels, one or more lanes are designated as counterflow lanes, meaning that the direction of traffic in those lanes can be reversed at any time. Special "lane control signals," placed above the roadway at regular distances, are used for this purpose, with one signal for each lane.

Like regular traffic lights, lane control signals around the world follow their own universal pattern, as specified in the Vienna Convention on Road Signs and Signals. Typical signals include a green downward arrow, used to indicate a lane which is open to traffic facing the signal, a red cross, which indicates a lane is either reserved for opposing traffic or closed to traffic in both directions, and a flashing amber circle, arrow or cross, indicating to traffic facing the signal to immediately clear the lane.

Unlike regular traffic lights, lane control signals either have one face each to indicate all lane conditions (the so-called "searchlight" configuration), or separate faces for each condition (as illustrated). Lane control signals of the latter type are always placed horizontally. Signals that may indicate other conditions for roads without counterflow lanes also exist, such as those that indicate different speed limits for different lanes.

Use as a rating mechanism

In the UK's British Civil Service and other government offices, traffic lights are used as a coding system for good or bad. For example, for the number of staff one has in relation to the workload, red would mean inadequate, amber would mean reasonable, and green would mean ideal.

External links


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