Traffic lights, which may also be known as stop lights, stoplight, traffic lamps, stop-and-go lights, robots or semaphore, are signaling devices positioned at road intersections, pedestrian crossings and other locations to control competing flows of traffic. Traffic lights have been installed in most cities around the world. They assign the right of way to road users by the use of lights in standard colors (Red - Amber - Green), using a universal color code (and a precise sequence, for those who are color blind).

The most common traffic lights consist of a set of three lights: red, amber, and green. When illuminated, the red light indicates for vehicles facing the light to stop; the amber indicates caution, either because lights are about to turn green or because lights are about to turn red (depending on the region of the world you are in); and the green light to proceed, (if it is safe to do so). There are some variations in the use and legislation of traffic lights, depending on the customs of a country and the special needs of a particular intersection. There may, for example, be special lights for pedestrians, bicycles, buses, trams, etc. and there may be special rules or sets of lights for traffic turning in a particular direction. Complex intersections may use any combination of these.

History

On 10 December 1868, the first traffic lights were installed outside the British Houses of Parliament in London, by the railway engineer J. P. Knight. They resembled railway signals of the time, with semaphore arms and red and green gas lamps for night use.The gas lantern was turned with a lever at its base so that the appropriate light faced traffic. Unfortunately, it exploded on 2 January 1869, injuring or killing the policeman who was operating it.

The modern electric traffic light is an American invention. As early as 1912 in Salt Lake City, Utah, policeman Lester Wire invented 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 East 105th Street and Euclid Avenue in Cleveland, Ohio. It had two colors, red and green, and a buzzer, based on the design of James Hoge, to provide a warning for color changes. The design by James Hoge allowed police and fire stations to control the signals in case of emergency. The first four-way, three-color traffic light was created by police officer William Potts in Detroit, Michigan in 1920. In 1923, Garrett Morgan patented a traffic signal device. Ashville, Ohio claims to be the location of the oldest working traffic light in the United States, used at an intersection of public roads until 1982 when it was moved to a local museum.

The first interconnected traffic signal system was installed in Salt Lake City 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. The first automatic experimental traffic lights in England were deployed in Wolverhampton in 1927.

The color of the traffic lights representing stop and go might be derived from those used to identify port (red) and starboard (green) in maritime rules governing right of way, where the vessel on the left must stop for the one crossing on the right.

Types of traffic lights

In many regions, traffic lights function differently or have different displays depending on available technology, traffic patterns, or other vehicles such as trolleys that also use the intersection. For example, some fixtures feature a flashing green light or more than one arrow lit at one time. An example of a flashing green light found in Canada, to notify left turning drivers that they have the right of way and that the opposing lanes and pedestrians will not be moving.

Three-set lights

The universal standard is for the red to be above the green, and if there is also an amber it is placed in the middle. If the three-set lights are mounted horizontally, the red will typically be to the left of the green. The standards apply whether the country drives on the left or the right, but the placement of the mountings on the road would be mirror images of the other.

Go-stop sequence

In most countries, the sequence is green (go), amber (prepare to stop), and red (stop). In Russia, Serbia, Austria, Israel, and parts of Canada and Mexico, the green light flashes for a few seconds before the amber light comes on. In New Zealand and Canada, amber officially means 'stop (unless it would cause an accident to do so)' but in practice, is treated as 'prepare to stop'.

Stop-go sequence

In most countries the sequence is red (stop), green (go). In some countries, the sequence is red (stop), yellow (prepare to go), green (go). In some places, such as the UK, Germany and Russia, the sequence is red (stop), red and amber (stop, prepare to go), green (go if clear).

Flashing yellow light

In some countries, a flashing yellow light means that intersection must be treated as non-signalized, other road signs such as "yield" or "major road" must be obeyed if present. The single flashing amber signal is used in the UK, Ireland and Australia at Pelican crossings. It is used in Serbia and the United States to mark places where greater attention is needed (dangerous crossings, sharp curves etc.). In Canada, a flashing amber light means "drive with caution" and is frequently combined with a flashing red light (meaning "stop") at four-way intersections. In many South East Asian countries (e.g. Thailand) and European countries, a flashing amber light indicates a driver may proceed cautiously across a junction where signals operate only at busy periods.

Flashing red light

A flashing red is usually treated as a stop sign.

Pedestrian crossing lights

Image:Sinal.JPG|Typical pedestrian crossing lightImage:Semaforopedonale.jpg|Pedestrian crossing light with loudspeaker for the blindImage:Semáforo regressivo poá.JPG|A crosswalk traffic signal in Poá, BrazilImage:Walklight phases anim.gif|Three-state pedestrian lights‎Image:Kiev-Verkeerslicht.jpg|Traffic light with counter (seconds till green light) in Kiev



Traffic lights for pedestrians normally have two main lights: a red light that means 'stop' and a green light that means 'go' (or, more correctly, 'proceed with caution'). There is usually a flashing phase (red in the US, Canada, Australia and New Zealand, green in Europe) that means 'complete your crossing'. In most locales in North America, the colors used are a red-orange ("Portland orange") for "stop/wait" and a bluish-white ("lunar") for "go." While the "walk" signal is generally a walking human figure, North American pedestrian signals usually show an upraised hand for "stop," while most other countries display a standing human figure. Some older American signals display the verbal commands "Walk" (lunar white or green) and "Don't Walk" or "Wait" (red-orange). US pedestrian signals are now standardized as being white and orange.

At selected pedestrian crossings in some countries, pedestrian traffic lights include a type of siren, beeper or warbler, which sounds in order to alert visually impaired pedestrians that it is safe to cross (In The Republic of Ireland this is the case at all Pedestrian crossings]]. These may be set to a timer and sound only at daytime to avoid annoying sleeping residents at night, or may be activated by a button. Some other intersections include a white strobe light mounted inside the red light that flashes every few seconds when the light is red. This is mainly used when a new traffic light is installed or where running a red light has proven to be a problem. Some also include tactile warnings, like a vibrating plate, or a rotating cone, to help people with hearing impairment or visual impairment cross the road and street.

Some pedestrian crossing lights are activated only after a pedestrian presses an activating button, while others operate automatically, and others still operate automatically but only at certain times of the day.

In some jurisdictions, at intersections where there are no pedestrian crossing lights in operation, the normal three-set lights also apply to pedestrians crossing the street.

Some pedestrian and roadway traffic signals are fitted with readouts showing a countdown until the next signal change.

Some traffic-light controlled junctions have a light sequence that stops all vehicular traffic at the junction at the same time, and gives pedestrians exclusive access to the intersection so that they can cross in any direction (including diagonally). This is known as a pedestrian scramble in some places.

Ampelmännchen pedestrian traffic signals have come to be seen as a nostalgic sign for the former German Democratic Republic.

Lights for public transport

Traffic lights for public transport often use signals that are distinct from those for private traffic. They can be letters, arrows or bars of white or colored light.

In Portland, Oregon, the tram signals feature a horizontal white bar and an orange vertical bar. Some systems use the letter B for buses, and T for trams.

In Russia traffic signals for public transport have four white lights that form the letter T. If the three top lamps are lit, this means "stop". If the bottom lamp and some lamps on the top row are lit, this means permission to go in a direction shown. If there are no tram junctions on an intersection, a simpler system of one amber signal in the form of letter T is used instead; the tram must proceed only when the signal is lit.

In North European countries the tram signals feature white lights of different forms: "S" for "stop", "—" for "caution" and arrows to permit passage in a given direction.



The Netherlands use a distinctive "negenoog" (nine-eyed) design shown on the top row of the diagram; bottom row signals are used in Belgium and France. The signals mean (from left to right): "go straight ahead", "go left", "go right", "go in any direction" (like the "green" of a normal traffic light), "stop, unless the emergency brake is needed" (equal to "yellow"), and "stop" (equal to "red").

Lights for cyclists

Image:Bielefeld_comb_ped_cycle.JPG|Combined Pedestrian and Bicycle DisplayImage:19-Sept-09_amber.jpg|Amber Bicycle LightImage:Berlin_small_lights_01.JPG|Small Bicycle Lights in Berlin

Traffic lights for cyclists use a symbol of a bicycle. In some jurisdictions, bicycle and pedestrian lights are shown on the same signal aspect. Where cycle signals are separate from pedestrian signals, most commonly red, amber and green are used to control the movements. In some jurisdictions, the amber aspect does not need to be shown.

In some countries, signals for cyclists are mounted at eye-height, and these are much smaller than ordinary signals.

Colors

The most common colors used in traffic lights are red, amber (yellow), and green. Red typically means stop or high level of danger; amber typically means caution; and green typically means proceed with care.

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 the UK, traffic lights typically have a white reflective border, which enables color blind users, during the hours of darkness, to distinguish the lights from other similarly-colored street or automobile lights, and to allow them to distinguish the lights by vertical position.

Turning signals and rules

[[Image:Traffic lights 4 states.svg|thumb|200px|Overview of UK 4-state traffic lights:

1. Stop

2. Stop

3. Go if clear

4. Stop]] In some instances, traffic may turn left (in left-driving jurisdictions) or right (in right-driving jurisdictions) after stopping at a red light, providing they give way to the pedestrians and other vehicles. In some places that generally disallow this, a sign next to the traffic light indicates that it is allowed at a particular intersection. Conversely, jurisdictions that generally allow this might forbid it at a particular intersection with a "no turn on red" sign, or put a green arrow to indicate specifically when a turn is allowed without having to yield to pedestrians (this is usually when traffic from the perpendicular street is making a 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 (a right-driving jurisdiction) concluded that allowing drivers to proceed straight on red after stopping, at specially posted T-intersections where the intersecting road went left only, was dangerous. Proceeding straight on red at T-intersections where the intersecting road went left only used to be legal in Mainland China, with right-hand traffic provided that such movement would not interfere with other traffic, but when the Road Traffic Safety Law of the People's Republic of China took effect on 1 May 2004, such movement was outlawed. In some other countries, the permission is indicated by a flashing amber arrow (cars do not have to stop but must give 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 and Canada (since opposing traffic is on the left). With dedicated left turn signals, a left-pointing arrow turns green when traffic may turn left without opposing traffic and pedestrian conflict, and turns red or disappears otherwise. Such a signal is referred to as a "protected" signal if it has its own red phase; a "permissive" signal does not have such a feature. Three standard versions of the permissive signal exist: One version is a horizontal bar with five lights - the green and yellow arrows are located between the standard green and yellow lights. A vertical 5-light bar holds the arrows underneath the standard green light (in this arrangement, the yellow arrow is sometimes omitted, leaving only the green arrow below the solid green light, or possibly an LED based device capable of showing both green and yellow arrows within a single lamp housing). Some newer LED turn arrows seen in parts of Canada are capable of multicolored animation. Such lights will often display a flashing and animated green or yellow arrow when the dedicated turn is allowed, but then transform into a red arrow on a white background with a red line through it, emphasising that the turn is no longer allowed. These lights will also often have the words "NO TURN" displayed, or an explanatory reason why the turn is not allowed, such as "TRAIN" in the case of a rail or light rail crossing. A third type is known as a "doghouse" or "cluster head" - a vertical column with the two normal lights is on the right side of the signal, a vertical column with the two arrows is located on the left, and the normal red signal is in the middle above the two columns. Cluster signals in Australia and New Zealand use six signals, the sixth being a red arrow that can operate separately from the standard red light. In a fourth type, sometimes seen at intersections in Ontario and Quebec, Canada, there is no dedicated left-turn lamp per se. Instead, the normal green lamp flashes rapidly, indicating permission to go straight as well as make a left turn in front of opposing traffic, which is being held by a steady red lamp. (This "advance green," or flashing green can be somewhat startling and confusing to drivers not familiar with this system. This also can cause confusion amongst visitors to British Columbia, where a flashing green signal denotes a pedestrian controlled intersection.) Another interesting practice seen at least in Ontario is that cars wishing to turn left that arrived after the left turn signal ended can do so during the amber phase, as long as there is enough time to make a safe turn.

A flashing amber arrow, which allows drivers to make left turns after giving way to oncoming traffic, is becoming more widespread in the United States, particularly in Oregon. In the normal sequence, a protected green left-turn arrow will first change to a solid amber arrow to indicate the end of the protected phase, then to a flashing amber arrow, which remains flashing until the standard green light changes to amber and red. In Oregon, the amber flashing arrow is usually housed in a separate light section from the steady amber arrow, in order to provide a visible position change. These generally take the form of four signal sections (green, amber, amber, red). On some newer signals, notably in the city of Bend, the green and flashing amber arrows emanate from the same light section through the use of a dual-color LED array, while the solid amber arrow is mounted above it.

Generally, a dedicated left-turn signal is illuminated at the beginning of the green phase of the green-yellow-red-green cycle. This is called a leading turn. This allows left-turn traffic, which often consists of just a few cars, to vacate the intersection quickly before giving priority to vehicles traveling straight. This increases the throughput of left-turn traffic while reducing the number of drivers, perhaps frustrated by long waits in heavy traffic for opposing traffic to clear, attempting to make an illegal left turn on red. A dedicated left-turn signal that appears at the end of the green phase is called a lagging turn. If there is no left-turn signal, the law requires one to yield to oncoming traffic and turn when the intersection is clear and it is safe to do so. Nevertheless, it is increasingly and disturbingly common in at least the U.S. to see drivers who do not yield in the absence of a dedicated signal, cutting off traffic that has right-of-way and is starting to head across the intersection. In the U.S., many older inner-city and rural areas do not have dedicated left-turn lights, while most newer suburban areas have them. Such lights tend to decrease the overall efficiency of the intersection as it becomes congested, although it makes intersections safer by reducing the risk of head-on collisions and may even speed up through traffic, but if a significant amount of traffic is turning, a dedicated turn signal helps eliminate congestion.

Some intersections with protected-turn signals occasionally have what is known as "yellow trap", "lag-trap", or "left turn trap" (in right-driving countries). It occurs at intersections where vehicles are permitted to make left turns on normal green lights. "Yellow trap" refers to situations when left-turning drivers are trapped in the intersection with a red light, while opposing traffic still has a green. In British Columbia, the law addresses this problem by giving a left-turning driver already in an intersection the right-of-way to make the turn once the intersection is otherwise clear, regardless of the traffic light state.

For example, an intersection has dedicated left-turn signals for traffic traveling north. The southbound traffic gets a red light so northbound traffic can make a left turn, but the straight-through northbound traffic continues to get a green light. A southbound driver who had entered the intersection earlier will now be in a predicament, since they have no idea whether traffic continuing straight for both directions is becoming red, or just their direction. The driver will now have to check the traffic light behind them, which is often impossible from the viewing angle of a driver's seat. This can also happen when emergency vehicles or railroads preempt normal signal operation. In the United States, signs reading "Oncoming traffic has extended green" or "Oncoming traffic may have extended green" must be posted at intersections where the "yellow trap" condition exists.

Although motorcycles and scooters in most jurisdictions follow the same traffic signal rules for left turns as do cars and trucks, some places, such as Taiwan, have different rules. In these areas, it is not permitted for such small and often hard-to-see vehicles to turn left in front of oncoming traffic on certain high-volume roads when there is no dedicated left-turn signal. Instead, in order to make a left turn, the rider moves to the right side of the road, travels through the first half of the intersection on green, then slows down and stops directly in front of the line of cars on the driver's right waiting to travel across the intersection, which are of course being held by a red light. There is often a white box painted on the road in this location to indicate where the riders should group. The rider turns the bike 90 degrees to the left from the original direction of travel and proceeds along with the line of cars when the red light turns green, completing the left turn. This procedure improves safety because the rider never has to cross oncoming traffic, which is particularly important given the much greater likelihood of injury when a cycle is hit by a car or truck. This system (called a "hook-turn") is also used at many intersections in the CBD of Melbourne, Australia, where either or both streets carry tramways. This is done so right-turning vehicles (Australia drives on the left) do not block the passage of trams. The system is being extended to the suburbs.

At intersections where no turns are allowed from any direction, the green light can be replaced with a green arrow pointing up.

In the UK, many lights (as shown in the image above) have "filters". These are green (usually in the shape of arrows) and, when lit, allow traffic in a certain direction at a junction to proceed, while holding back other traffic. Sometimes there are special lanes for such traffic, and it is important to prepare to be in the correct lane before the junction. Filters can confuse the unwary for this reason, and also because seeing a green light they start to proceed before realising it does not apply to them. There is no default "Left turn on Red" system in the UK.

Lane control

Lane control lights are a specific type of traffic light used to manage traffic on a multi-way road or highway. Typically they allow or forbid traffic to use or more of the available lanes by the use of green lights or arrows (to permit) or by red lights or crosses (to prohibit).

Special provisions

Traffic light failure in most jurisdictions must be handled by drivers as a priority-to-the-right intersection in both drive-on-the-left Australia and some states of the mainly drive-on-the-right Europe, or an all-way stop elsewhere, pending the arrival of a police officer to direct traffic.

Some jurisdictions, however, have additional right-of-way signs mounted above, below or next to the traffic lights; these take effect when the lights are no longer active. (In Italy as well as some jurisdictions in the US, traffic lights inactive at nighttime emit an amber-colored flashing signal in directions owing priority while the intersecting street emit a flashing red light, requiring drivers to stop before proceeding. In Germany the priority directions will not be illuminated while the intersecting streets will be shown a flashing yellow signal.) In the UK and parts of North America, drivers simply treat the junction as being uncontrolled when traffic lights fail, giving way as appropriate, unless a police officer is present. In much of the United States failed traffic signals must be treated as all-way stop intersections.

Dummy lights

A dummy light is a traffic light that stands on a pedestal in the middle of an intersection. There are at least three that still operate in the United States today, all located in New York State: Beacon, Canajoharie and Croton-on-Hudson. There have been number of requests in recent years for these traffic lights to be removed due to safety concerns, but the historic value has kept these landmarks at their original locations.

Technology

Optics and lighting

In the mid 1990s, cost-effective traffic light lamps using light-emitting diodes (LEDs) were developed; prior to this date traffic lights were designed using incandescent or halogen light bulbs. Unlike the incandescent-based lamps, which use a single large bulb, the LED-based lamps consist of an array of LED elements, arranged in various patterns. When viewed from a distance, the array appears as a continuous light source.

LED-based lamps (or 'lenses') have numerous advantages over incandescent lamps; among them are:

• Much greater energy efficiency (can be solar-powered).
• Much longer lifetime between replacement, measured in years rather than months. Part of the longer lifetime is due to the fact that some light is still displayed even if some of the LEDs in the array are dead.
• Brighter illumination with better contrast against direct sunlight, also called 'phantom light'.
• The ability to display multiple colors and patterns from the same lamp. Individual LED elements can be enabled or disabled, and different color LEDs can be mixed in the same lamp
• Much faster switching.
• Instead of sudden burn-out like incandescent-based lights, LEDs start to gradually dim when they wear out, warning transportation maintenance departments well in advance as to when to change the light. Occasionally, particularly in green LED units, segments prone to failure will flicker rapidly beforehand.

The operational expenses of LED-based signals are far lower than equivalent incandescent-based lights. As a result, most new traffic light deployments in the United States, Canada and elsewhere have been implemented using LED-based lamps; in addition many existing deployments of incandescent traffic lights are being replaced. Sometimes, only the red and green lamps are replaced, leaving the yellow lamp as an incandescent, since yellow lamps are rarely on compared to the red and green lamps. In 2006, Edmonton, Alberta, Canada completed a total refit to LED-based lamps in the city's over 12,000 intersections and all pedestrian crosswalks. Many of the more exotic traffic signals discussed on this page would not be possible to construct without using LED technology. However, color-changing LEDs are in their infancy and may surpass the multi-color array technology.

Another new LED technology is the use of CLS (Central Light Source) optics. These comprise around 7 high-output LEDs (sometimes 1 watt) at the rear of the lens, with a diffuser to even out and enlarge the light. This gives a uniform appearance, more like traditional halogen or incandescent luminaries.

Replacing halogen or incandescent reflector and bulb assemblies behind the lens with an LED array can give the same effect. This also has its benefits: minimal disruption, minimal work, minimal cost and the reduced need to replace the entire signal head (housing).

Programmable Visibility Signals

such as the 3M High Visibility Signal and McCain Programmable Visibility signal, utilize light diffusing optics and a powerful fresnel lens to create the signal indication. Lit via a powerful 150W PAR46 sealed-beam lamp, the light from the lamp in these "programmable visibility" signals passes through a set of two glass lenses at the back of the signal. The first lens, a frosted glass diffusing lens, diffuses the light into a uniform ball of light around five inches in diameter. The light then passes through a nearly identical lens known as an optical limiter (3M's definition of the lens itself), also known as a "programming lens", also five inches in diameter.

Using a special aluminium foil-based adhesive tape, these signals are "masked" or programmed via the programming lens so that only certain lanes of traffic will view the indication. At the front of these programmable visibility signals is a 12" fresnel lens, each lens tinted to meet United States Institute of Transportation Engineers (ITE) chromaticity and luminance standards. The fresnel lens (3M utilized a square lens masked to create a ball indication, McCain used a circular lens itself) amplifies the light output created by the lamp, and creates a uniform display of light for the lane in which it is intended. These signals were first developed by the 3M Company in the late 1960s, and were wildly popular in the late 1970s as traffic density increased.

Typical applications for these signals were skewed intersections, specific multi-lane control, left-turn pocket signals or other areas where complex traffic situations existed.

Conventional lighting systems

Conventional traffic signal lighting, still common in some areas, utilizes a standard light bulb. Typically, a 67 watt, 69 watt, or 115 watt medium-base (household lamp in the U.S) light bulb provides the illumination. Light then bounces off a mirrored glass or polished aluminium reflector bowl, and out through a polycarbonate plastic or glass signal lens. In some signals, these lenses were cut to include a specific refracting pattern. Crouse-Hinds is one notable company for this practice. In the 1930s throughout the 1950s, they utilized a beaded prismatic lens with a "smiley" pattern embossed into the bottom of each lens.

Light design

In the United States, traffic lights are currently designed with lights approximately in diameter. Previously the standard had been , however those are slowly being phased out in favor of the larger and more visible 12 inch lights. Variations used have also included a hybrid design, which had one or more 12 inch lights along with one or more lights of on the same light. For example, these "12-8-8" (along with 8-8-8) lights are standard in most jurisdictions in Ontario, Manitoba, and British Columbia (that, is, the red light is 12 and others 8, making the red more prominent).

In the United Kingdom, 12 inch lights were implemented only with Mellor Design Signal heads designed by David Mellor. These were designed for symbolic optics to compensate for the light loss caused by the symbol. With the invention of anti-phantom, highly visible SIRA lenses, lights of could be designed to give the same output as plain lenses, so a larger surface area was unnecessary. Consequently lights of are no longer approved for use in the UK and all lights installed on new installations have to be in accordance with TSRGD (Traffic Signs Regulations and General Directions). Exemptions are made for temporary or replacement signals.

In the state of Victoria, Australia, lights used to have long visors with a slanted cut on the side nearest the road (for approach side) and none for the opposite side. These lights also had a sort of horizontal filter covering the upper half of the visor, except on arrows. This design eliminated or minimised washout of the light by a low-lying sun. This type of design has yet to surface outside of Victoria. LED-based lights, however, do not follow this standard, adopting the national standard of having a short visor and cut on both sides, although the cut is also applied to the lights on the opposite side of the junction. Some installations do not have cuts on the opposite side. There exists at least 1 LED-based light using the visor and filter set of its predecessor.

Control and coordination

The normal function of traffic lights requires sophisticated control and coordination to ensure that traffic moves as smoothly and safely as possible and that pedestrians are protected when they cross the roads.

Mounting

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 horizontal poles or wires strung over the roadway, 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.



Japan, South Korea, Taiwan, Thailand and Croatia, along with some jurisdictions in the U.S. (including all in the State of New Mexico),some places in Canada and Mexico mount lights with their multiple faces arranged horizontally, often with supplemental vertical signals on the side, while others use vertical signals almost exclusively. Horizontal signals have consistent orientation, like their vertical counterparts. Often, supplemental curb pedestal mounts, intended to support a signal for a different approach road, are used when primary signals are partially obscured due to structures such as overpasses, approaches around a building that obscures the primary signal mountings, and unusual approach geometry. In Florida, horizontal signals mounted on poles, known as "mast arms", are in wide use due to their lower wind profile, important for minimising hurricane damage. The horizontally traffic light position is very common in Houston on most intersections.

California is particularly rigorous 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. Those numbers must be multiplied 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. One example of the Fresnel lens application common in the USA is the 3M Model 131 traffic signal (dubbed the "Programmed Visibility" signal). Although 3M has recently discontinued the M-131, it became a popular traffic signal for skewed or complex intersections. Today, McCain Traffic Systems (a traffic signal manufacturing company based in Vista, California) is the only U.S.-based manufacturer producing true "programmable visibility" traffic signals. In addition to aiming, Fresnel lenses, and louvers, visors and back panels are also useful in areas where sunlight would diminish the contrast and visibility of a signal face.

Traffic signals in most areas of Europe are located at the stop line on same side of the intersection as the approaching traffic (there being both right- and left-hand traffic) and are often mounted overhead as well as on side of the road. At particularly busy junctions for freight, higher lights may be mounted specifically for trucks. The stop line alignment is done to prevent vehicles blocking any crosswalk blocking and allow for better pedestrian traffic flow. There may also be a special area a few metres in advance of the stop line where cyclists may legally wait but not motor vehicles; this advanced stop line is often painted with a different road surface with greater friction and a high color, both for the benefit of cyclists and for other vehicles. The traffic lights are mounted so that cyclists can still see them.

In North America, there is often a pole-mounted signal on the same side of the intersection, but additional pole-mounted and overhead signals are usually mounted on the far side of the intersection for better visibility. In some areas, signals facing all four directions are hung directly over the intersection on a wire strung diagonally over the intersection.

In Lloydminster, a city straddling the provincial border between Alberta and Saskatchewan, an unusual jurisdictional arrangement can be found: motorists proceeding east at the intersection situated on the border must, while in Alberta, stop for a red signal located in Saskatchewan; the reverse applies for westbound motorists. If such a motorist were to disregard the signal and collide with a vehicle proceeding from that motorist's left, the collision would occur in a province other than the one where the traffic control device is located.

Implementation

According to transportation engineers, traffic lights can have both positive and negative effects on traffic safety and traffic flow. The separation of conflicting streams of traffic in time can reduce the chances of right-angle collisions. But also the frequency of rear-end crashes can be increased by the installation of traffic lights, and they can adversely affect the safety of bicycle and pedestrian traffic. They can increase the traffic capacity at intersections, but can also result in excessive traffic delay. Hans Monderman, the innovative Dutch traffic engineer, and pioneer of shared space schemes, was sceptical of their role, and is quoted as having said of them: "We only want traffic lights where they are useful and I haven't found anywhere where they are useful yet".

Traffic signal warrants

Traffic signals have strengths and weaknesses that must be considered when deciding whether to install them. Signaled intersections can reduce delay for side road traffic and reduce the occurrence of collisions by turning traffic and cross traffic. But they may also cause delay for traffic on the main road, and often increase rear-end collisions by up to 50%. Since right-angled and turn-against-traffic collisions are more likely to result in injuries, this is often an acceptable tradeoff.

Criteria have been developed to help ensure that new signals are installed only where they will do more good than harm. In the United States these criteria are called warrants, and are found in the Manual on Uniform Traffic Control Devices (MUTCD), the Federal regulation covering the use of signs, pavement markings, traffic signals, and similar devices.

In the US MUTCD, there are eight warrants for traffic signals:

• Eight-Hour Vehicular Volume. Traffic volume must exceed prescribed minima for eight hours of each weekday.
• Four-Hour Vehicular Volume. Traffic volume must exceed prescribed minima for four hours of each weekday.
• Peak Hour Volume or Delay. This is applied only in unusual cases, such as office parks, industrial complexes, or park and ride lots that attract or discharge large numbers of vehicles in a short time, and, for a minimum of one hour of an average day. The side road traffic suffers undue delay when entering or crossing the major street.
• Pedestrian Volume, if the traffic volume on a major street is so heavy that pedestrians experience excessive delay in attempting to cross it
• School Crossing, if there not enough safe gaps in traffic for school children to safely cross the street
• Coordinated Signal System, for places where adjacent traffic control signals do not keep traffic grouped together efficiently.
• Crash Experience. The volumes in the eight- and four-hour warrants may be reduced if five or more right-angle and cross traffic turn collisions have happened at the intersection in a twelve month period.
• Roadway Network. Installing a traffic control signal at some intersections might be justified to encourage concentration and organization of traffic flow on a roadway network.

An intersection should meet one or more of these warrants before a signal is installed. However, meeting one or more warrants does not require the installation of a traffic signal, only suggests that they may be suitable. It could be that a roundabout would work better there. There may be other unconsidered conditions that lead to traffic engineers to conclude a signal is undesirable. For example, they may decide not to install a signal at an intersection if traffic stopped by it will back up and block another, more heavily trafficked intersection. Also, if a signal meets only the peak hour warrant, the advantages during that time may not outweigh the disadvantages during the rest of the day.

Between 1979 and 1988, the city of Philadelphia removed 199 signals that did not meet any of the warrants. On average, the intersections had 24% fewer crashes after the unwarranted signals were removed.

Preemption

Some regions have signals that are interruptible, giving priority to special traffic. Such traffic light preemption is usually reserved for emergency vehicles such as fire apparatus, ambulances, and police squad cars, though sometimes mass transit vehicles including buses and light rail trains can interrupt lights. Most of the systems operate with small transmitters that send radio waves, infrared signals, or strobe light signals that are received by a sensor on or near the traffic lights. Some systems use audio detection, where a certain type of siren must be used and detected by a receiver on the traffic light structure.

Upon activation the normal traffic light cycle is suspended and replaced by the "preemption sequence": the traffic lights to all approaches to the intersection are switched to "red" with the exception of the light for the vehicle that has triggered the preemption sequence. Sometimes, an additional signal light is placed nearby to indicate to the preempting vehicle that the preempting sequence has been activated and to warn other motorists of the approach of an emergency vehicle. The normal traffic light cycle resumes after the sensor has been passed by the vehicle that triggered the preemption.

In lieu of pre-emptive mechanisms, in most jurisdictions, emergency vehicles are not required to respect traffic lights, but must activate their own emergency lights when crossing an intersection against the light, in order to alert oncoming drivers to the preemption.

Legal implications

In virtually all jurisdictions in which they are used, it is an offence for motorists (and other road users) to disregard the instructions of traffic lights (or other traffic control devices). Exceptionally, it is not an offence for pedestrians to cross against a red light in the United Kingdom, where pedestrian lights officially give advice, rather than an instruction, although UK pedestrians do commit an offence if they cross a road against the signals of a police officer controlling traffic.

The most common infraction associated with traffic lights is failing to stop for a red light (in some jurisdictions, running an amber light can also incur a penalty). Enforcement of traffic lights varies from jurisdiction to jurisdiction; some places are extremely strict. Other locales are infamous for traffic lights being routinely ignored by motorists, with no serious attempts by law enforcement to alter the situation.

Jurisdictions differ somewhat on how to deal with "red light running" — attempts by motorists to race to an intersection while facing a yellow light, in an attempt to beat the red. In some locales, as long as the light is yellow when the motorist enters the intersection, no offense has been committed; in others, if the light turns red at any time before the motorist clears the intersection, then an offense occurs. In Oregon and other places, a stricter standard applies-—running an amber light is an offense, unless the motorist is unable to stop safely. This standard has been criticized as ambiguous and difficult to enforce (red light cameras in Oregon are activated only if a motorist enters the intersection on a red). Red light cameras in NSW, Australia are activated only if a motorist enters an intersection 0.3 seconds after the light has turned red.

In some jurisdictions (such as Toronto, Washington D.C., New York City), and the state of California, there are ordinances against "gridlocking." Any motorist who enters an intersection (even if on a green light) but does not ensure that one can proceed through the intersection and gets stuck in the middle of the intersection (when traffic ahead fails to proceed), and remains there after the light turns red (thus blocking traffic coming from other directions) may receive a citation. The definition of the intersection area is that square where the two streets overlap marked by the inner lines of each crosswalk. (Occupying the space inside the crosswalk lines is itself a traffic infraction, but different from gridlocking.) This gives the meaning to the anti-gridlock slogan "Don't Block the Box." This is sometimes used as a justification for making a turn across the opposing travel lanes on a red light at a busy intersection, by pulling partway into the intersection at a green light waiting to perform the turn, and, if oncoming traffic is not abated before the light changes to red, proceeding to turn once the light has turned red and opposing traffic has stopped. This means that at busy junctions without a protected green arrow for turning traffic, one turns after the light turns red. This maneuver is commonly referred to as "occupying the intersection" or "being legally allowed to complete one's turn". In some jurisdictions, including most American states, a vehicle already in the intersection when the light turns red legally has the right of way, and vehicles who have green must yield to the vehicle in the intersection.

Some local driving traditions may be legally questionable. A prime example is the Pittsburgh left. Although failing to yield to oncoming traffic while navigating a turn is a serious traffic violation and is prohibited in the Commonwealth of Pennsylvania, many drivers will make a Pittsburgh left. The Pittsburgh Left: a driver waiting at a red light to turn left is allowed to turn by oncoming traffic when the light turns green, instead of yielding the right-of-way as the law requires. Although illegal, such behavior is regarded as not only customary but also polite in the city of Pittsburgh.

Similarly, in Sackville, New Brunswick and most of eastern Massachusetts it is customary for through traffic to voluntarily yield to the first oncoming left-turning vehicle to allow it to perform a Pittsburgh left. This is similar to a hook turn performed in Melbourne, Australia, which is legal at signed intersections.

Enforcement of traffic lights is done in one of several ways:

• By police officers observing traffic through the light, and pulling over and issuing citations to motorists who violate the signal.
• As a result of an accident investigation, if it is determined that one or more motorists ran the red light — even if the incident was not observed by a police officer.
• With red light cameras.

Red light cameras

In some areas, red light cameras are used. An automated camera is connected to the triggering mechanism for the corresponding traffic light, which is targeted to photograph any vehicle, which crosses against the light. The driver or owner (depending on local laws) of a vehicle so photographed can then be fined for violating traffic laws.

Light timing length

The length of amber lights can differ, for example in many places the length of an amber light is usually 5 seconds, but elsewhere it may be as little as 3 seconds considerably reducing the time the driver has to react. It is typical for these times to vary according to the set speed limit, with longer times given for higher limits.

The time from when a red light is displayed and when a cross street is given a green light is usually based on the physical size of the intersection itself; this intervening period is called the all-red time. A typical all-red time is 2 sec to allow cars to clear the intersection. In a wider intersection, such as 4 lane road or highway intersection, the all-red time may be as much as 4 seconds, allowing drivers who could not stop at the amber light enough time to clear the intersection without causing a collision.

In other contexts

The symbolism of a traffic light (and the meanings of the three primary colors used in traffic lights) are frequently found in many other contexts. Since they are often used as single spots of color without the context of vertical position, they are typically not comprehensible to up to one in ten males who are color blind.

Traffic lights have also been used in pieces of artwork, particularly the Traffic Light tree in London, UK.

Racing

Automobile racing circuits can also use standard traffic signals to indicate to racing car drivers the status of racing. On an oval track, four sets may be used, two facing a straight-away and two facing the middle of the 180 degree turn between straight-aways. Green would indicate racing is under way, while yellow would indicate to slow or while following a pace car; red would indicate to stop, probably for emergency reasons.

Scuderia Ferrari, a Formula One racing team, formerly used a traffic light system during their pit stops to signal to their drivers to when to leave the pits. The red light was on when the tires were being changed and fuel was being added, yellow was on when the tires were changed, and green was on when all work was completed. The system is (usually) completely automatic. However, the system was withdrawn after the 2008 Singapore Grand Prix, due to the fact that it heavily delayed Felipe Massa during the race, when he was in the lead. Usually, the system was automatic, but heavy traffic in the pit lane forced the team to operate it manually. A mechanic accidentally pressed the green light button when the fuel hose was still attached to the car, causing Massa to drive off, towing the fuel hose along. Additionally, Massa drove into the path of Adrian Sutil, earning him a penalty. He finally stopped at the end of the pit lane, forcing Ferrari's mechanics to sprint down the whole of the pit lane to remove the hose. As a result of this, and the penalty he also incurred, Massa finished 13th. Ferrari decided to use a traditional "lollipop" for the remainder of the 2008 season.

As a rating mechanism

The colors red, amber and green are often used as a simple-to-understand rating system for products and processes. It may be extended by analogy to provide a greater range of intermediate colors, with red and green at the extremes.

See also

• Pedestrian crossing
• Level crossing
• Traffic optimization
• Ampelmännchen
• Yellow trap

Notes

1. Neato Stuff At the Ashville Museum. Ashville Area Heritage Society. Retrieved 2008-04-16.
2. Section 1.5: "System Evolution", Traffic Control Systems Handbook, Federal Highway Administration Report FHWA-HOP-06-006, October 2005.
3. Wolverhampton's Blue Plaques. Wolverhampton History & Heritage Society.
4. Accessed: 05 Aug 2008
5. National Cooperative Highway Research Program, Crash Reduction Factors for Traffic Engineering and Intelligent Transportation System (ITS) Improvements: State-of-Knowledge Report, November 2005, Table 3
6. Chapter 4F of the Manual on Uniform Traffic Control Devices
7. NSW RTA Road users' handbook

References

• City of Beacon. Canajoharie, New York: Credits. Retrieved Sep. 30, 2008.
• Croton-on-Hudson Historical Society. Canajoharie, New York: Credits. Retrieved Sep. 30, 2008.
• Villages of Canajoharie & Palatine Bridge. Canajoharie, New York: Credits. Retrieved Sep. 30, 2008.

External links

• Animations of various US signal phasings
• Traffic Lights by Ed Pegg, Jr., Wolfram Demonstrations Project: demonstrates that there are "seven distinct light cycles that will prevent collisions."
• Animations of various US signal phasings
• Micro Controller based Traffic Light Controller.