Traffic light

This article is about a traffic control device. For the type of political coalition, see Traffic light coalition.

"Traffic Signal" redirects here. For the 2007 Bollywood film, see Traffic Signal (film).

Traffic lights can have several additional lights for filter turns or bus lanes. This one in Warrington, also shows the distinctive red + amber combination seen in the UK.

The traffic light, also known as traffic signal, stop light, traffic lamp, stop-and-go lights, robot or semaphore, is a signaling device positioned at a road intersection, pedestrian crossing, or other location. Its purpose is to indicate, using a series of colours (Red - Amber - Green), the correct moment to stop, drive, ride or walk, using a universal colour code (and a precise sequence, for those who are colour blind).

Introduction

Crosswalk signal feature showing a female pictogram

Traffic lights for vehicles usually contain three lamps: red, orange or yellow (officially amber), and green. 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 is treated as a regular stop sign.

Traffic lights for pedestrians normally have two main lights—a red light that means 'stop' and a green (or sometimes white for pedestrians) light that means 'go' (or, more correctly, 'proceed with caution').

The use of these colours are thought to originate from nautical right-of-way. 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 colour blindness.

Traffic lights for special vehicles (such as buses or trams) may 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 green (go), amber (prepare to stop), and red (stop). In most systems, however, when red changes to green, both red and amber are lit. It is customary for drivers to select neutral and/or use the handbrake at red lights; the additional phase gives the driver time to select first gear or release the handbrake before the light turns green).

In the UK, 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'. In Russia and Serbia, the green light flashes for a few seconds before the amber light comes on. The single flashing amber signal is used in the UK, Ireland and Australia at Pelican crossings. Also it is used in Serbia to mark places where bigger attention is needed (dangerous crossings, sharp curves etc.).

Turning signals and rules

In some instances, traffic may turn right after stopping on a red (left in left-driving countries), providing they give way to the pedestrians and other vehicles. In some cases which generally disallow this, a 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 went only left, was dangerous. Proceeding straight on red at T-intersections where the intersecting road went only left was once 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. [1]. 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 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). 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. 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 "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.

Generally, a dedicated left-turn signal is illuminated at the beginning of the green phase of the green-yellow-red-green cycle. This allows left-turn traffic, which often consists of just a few cars, to vacate the intersection quickly before giving priority to vehicles travelling 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. 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.

Pedestrian crossing light with loudspeaker for the blind.

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.

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 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.

A traffic signal in Halifax, Nova Scotia with specially-shaped lights to assist people with colour blindness.

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 Germany and Italy traffic lights inactive at nighttime emit an amber-coloured flashing signal in directions owing priority.) In the UK and 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 1999, concerned that some traffic lights would fail as a result of the Y2K bug, some jurisdictions installed emergency unfoldable stop signs at intersections .

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. These may be set to a timer and only sound at day time, to avoid annoying residents. Some other intersections include a white strobe light mounted inside the red light that flashes every few seconds when the light is red. (See other comments on red with white strobe later in this article.) Some also include tactile warnings, like a vibrating plate, or a rotating cone, to help deafblind people cross the road and street

Mounting

File:Stevenscreekblvdtrafficlight.jpg

Typical example of traffic lights mounting in California.

In Ontario, traffic lights may often be mounted on telephone poles.

Road Construction may require the use of temporary lights.

Sometimes many lights are mounted on the same display, as with these lights in Melbourne.

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, and Mexico along with some jurisdictions in the U.S. and Canada 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.[2] 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.

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 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 minimise 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 fresnel example common in the USA is known as a 3M "Program Head", although 3M has recently discontinued the line. Shades and back panels are also useful in areas where sunlight would diminish the contrast and visibility of a signal face.

Animated traffic signal

Traffic signals in most areas of Europe are located at the stop line on same side of the intersection as the approaching traffic and are often mounted overhead as well as on the right and left sides of the road. The stop line alignment is done to prevent crosswalk blocking and allow for better pedestrian traffic flow. 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.

History

In the 1920s, after continued destruction of a standard traffic light in the then-predominantly Irish neighborhood of Tipperary Hill, the City of Syracuse, New York in the United States gave up and installed a traffic light with green on the top. Residents of Irish descent had objected to the fact that the "British" red was placed above the "Irish" green. However, this signal configuration can be a potential hazard to color-blind drivers.

Marshalite traffic signal. Formerly fitted in various intersections in Melbourne, indicating how much time remained before a signal change.

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 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 105th Street and Euclid Avenue in Cleveland, Ohio. It had two colours, red and green, and a buzzer, based on the design of James Hoge, to provide a warning for colour changes. The design by James Hoge (USPTO # 1251666 Sept. 22, 1913) allowed Police and Fire stations to control the signals in case of emergency. The first four-way, three-colour traffic light was created by police officer William Potts in Detroit in 1920. In 1923, Garrett Morgan patented a traffic signal device, although it was not a precursor of the modern traffic light. 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.

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

The color of the traffic lights representing stop and go are likely 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.

Technology

Optics and lighting

A modern British traffic light in Portsmouth.

LED traffic lights in Örnsköldsvik, Sweden.

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. Some of the longer lifetime is due to the fact that the light is an array which allows the light to be used 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 colours and patterns from the same lamp. Individual LED elements can be enabled or disabled, and different colour LEDs can be mixed in the same lamp.
• Much faster switching.
• Instead of a sudden burn-out like incandescent-based lights, LEDs start to gradually dim out when they wear out, allowing transportation maintenance departments to be warned well in advance as to when to change the light. Occasionally, particularly in green LED lenses, 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. 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, colour-changing LEDs are in their infancy and may surpass the multi-colour array technology.

In some areas, LED-based signals have been fitted (or retrofitted) with special Fresnel lenses (Programmed Visibility or 'PV' lenses) and/or diffusers to limit the line of sight to a single lane. These signals typically have a "projector"-like visibility; and maintain an intentionally limited range of view. Because the LED lights don't generate a significant amount of heat, heaters may be necessary in areas which receive snow, where snow can accumulate within the lens area and limit the visibility of the indications.

12-8-8 Traffic light design in California.

Another new LED technology is the use of CLS (Central Light Source) optics. These are comprised of (usually) around 7 high-output LEDs (sometimes 1 watt) at the rear of the lens, with a defuser to even out and enlarge the light. This sometimes gives a uniformed 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).

Light design

In the United States, traffic lights are currently designed with approximately 12 inch in diameter lenses/LED collections for the red, yellow and green lights. Previously the standard had been 8 inch lights, 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 lens along with one or more 8 inch lenses on the same light. For example, these "12-8-8" (along with 8-8-8) lights continue to be used throughout Ontario as the standard at less-busy intersections, although very few cities, like Hamilton have virtually all "12-8-8" lights only.

In the United Kingdom, 300mm (or 12 inch) optics were only implemented with Mellor (designed by David Mellor) Design Signal heads for symbolic optics only (as the light intensity was reduced by the symbol, and so the lens was made larger to compensate), however with the invention of anti-phantom, highly visable SIRA lenses, 200mm (8 inch) symbolic aspects produce the same light output than plain lenses, so a larger surface area is not needed, and thus 300mm lenses are now banned and all lenses installed on new installations have to be 200mm in accordance with TSRGD (Traffic Signs Regulations and General Directions). Exempt from this rule is Temporary or replacement signals, however no UK signal companies still produce 300mm optic signals.

Controller system

Computerised traffic control box

A traffic signal is typically controlled by a controller inside a cabinet mounted on a concrete pad. Although some electro-mechanical controllers are still in use (New York City still has over 11,000), modern traffic controllers are solid state. The cabinet typically contains a power panel, to distribute electrical power in the cabinet; a detector interface panel, to connect to loop detectors and other detectors; detector amplifiers; the controller itself; a conflict monitor unit; flash transfer relays; a police panel, to allow the police to disable the signal; and other components.

In the United States, controllers are standardised by the NEMA, which sets standards for connectors, operating limits, and intervals. The TS-1 standard was introduced in 1976 for the first generation of solid-state controllers.

Traffic controllers use the concept of phases, which are directions of movement lumped together. For instance, a simple intersection may have two phases: North/South, and East/West. A 4-way intersection with independent control for each direction and each left-turn, will have eight phases. Controllers also use rings; each ring is an of independent timing sequences. For example, with a dual-ring controller, opposing left-turn arrows may turn red independently, depending on the amount of traffic. Thus, a typical controller is an 8-phase, dual ring control.

Solid state controllers are required to have an independent conflict monitor unit (CMU), which ensures fail-safe operation. The CMU monitors the outputs of the controller, and if a fault is detected, the CMU uses the flash transfer relays to put the intersection to FLASH, with all red lights flashing, rather than displaying a potentially hazardous combination of signals. The CMU is programmed with the allowable combinations of lights, and will detect if the controller gives conflicting directions a green signal, for instance.

In the late 1990s, a national standardisation effort known as the Advanced transportation controller (ATC) was undertaken in the United States by the Institute of Transportation Engineers. The project attempts to create a single national standard for traffic light controllers. The standardisation effort is part of the National Intelligent transportation system program funded by various highway bills, starting with ISTEA in 1991, followed by TEA-21, and subsequent bills. The controllers will communicate using National Transportation Communications for ITS Protocol (NTCIP), based on Internet Protocol, ISO/OSI, and ASN.1.

Pedestrian scrambles

Main article: Pedestrian scramble

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 the intersection diagonally. 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. There is also a beeping sound to help the pedestrians. 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, the signals show both red and amber signals in all directions for this.

Control and coordination

Traffic signals must be instructed when to change phase. They can also be coordinated so that the phase changes called for occur in some relationship with nearby signals.

Pedestrian traffic signal in Taiwan, featuring a "Walking green man" below a countdown display where the "Red Man" once stood.

Fixed time control

Traffic signal phase changes are based on one of three systems: pre-timed, semi-actuated, and fully-actuated. The simplest control system uses a timer (fixed-time): each phase of the signal lasts for a specific duration before the next phase occurs; this pattern repeats itself regardless of traffic. Many older traffic light installations still use these, and timer-based signals are effective in one way grids where it is often possible to coordinate the traffic lights to the posted speed limit. They are however quite disadvantageous when the signal timing of an intersection would profit from being adapted to the dominant flows changing over the time of the day. In China, Singapore, Malaysia, Vietnam and Taiwan, pedestrian and roadway traffic signals may be fitted with readouts showing the countdown until the next signal change. In New Zealand, a few countdown readouts have been introduced (eg in Upper Hutt).

A crosswalk traffic signal in Kuala Lumpur with a countdown display similar to the one in Taiwan shown above. The only difference is that it has a display in the middle which shows the meanings of each phase in Malay.

Dynamic control

More sophisticated control systems use electronic detector loops, which are sensors buried in the pavement to detect the presence of traffic waiting at the light, and thus can avoid giving the green light to an empty road while motorists on a different route are stopped. A timer is frequently used as a backup in case the sensors fail; an additional problem with sensor-based systems is that they may fail to detect vehicles such as motorcycles or bicycles and cause them to wait forever (or at least until a detectable vehicle also comes to wait for the light). The sensor loops typically work in the same fashion as metal detectors; small vehicles or those with low metal content may fail to be detected.

It is sometimes more advantageous and cost effective to install over-roadway sensors than cutting the road and embedding inductive loops. These technologies include video image processors, sensors that use EM waves, or acoustic sensors to detect the presence of vehicles at the intersection waiting for right of way. These over-roadway sensors are more favourable than in-roadway sensors because they are immune to the natural degradation associated with paved right-of-way, competitively priced to install in terms of monetary and labour cost and danger to installation personnel, and have the capacity to act as real-time traffic management devices, act as multi-lane detectors, and collect data types not available from in-roadway sensors

It is also commonplace to alter the control strategy of a traffic light based on the time of day and day of the week, or for other special circumstances (such as a major event causing unusual demand at an intersection).

Coordinated control

Attempts are often made to place traffic signals on a coordinated system so that drivers encounter long strings of green lights. The distinction between coordinated signals and synchronised signals is very important. Synchronised signals all change at the same time and are only used in special instances or in older systems. Coordinated systems are controlled from a master controller and are set up so lights "cascade" in sequence so platoons of vehicles can proceed through a continuous series of green lights. A graphical representation of phase state on a two-axis plane of distance versus time clearly shows a "green band" that has been established based on signalised intersection spacing and expected vehicle speeds. In some countries (e.g. Germany, France and The Netherlands), this "green band" system is used to limit speeds in certain areas. Lights are timed in such a way that motorists can drive through without stopping if their speed is lower than a given limit, mostly 50 km/h (30 mph) in urban areas. This system is known as "grüne Welle" in German, "vague verte" in French, or "groene golf" in Dutch (English: "green wave").

In modern coordinated signal systems, it is possible for drivers to travel long distances without encountering a red light. This coordination 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 coordination, however. On the other hand, some traffic signals are coordinated to prevent drivers from encountering a long string of green lights. This practice discourages high volumes of traffic by inducing delay yet preventing congestion. Speed is self-regulated in coordinated signal systems; drivers travelling too fast will arrive on a red indication and end up stopping, drivers travelling too slowly will not arrive at the next signal in time to utilise the green indication. In synchronised systems, however, drivers will often use excessive speed in order to "make" as many lights as possible.

This traffic light in Khobar, Saudi Arabia is video camera-actuated (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. Non-actuated sensors occasionally impede traffic by detecting a lull and turning red just as cars arrive from the previous light. The most high-end systems use dozens of sensors and cost hundreds of thousands 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 in case the signals fail or are turned off. In the latter case there is a flashing amber light to support the yield or stop sign. Some lights outside of fire or rescue stations have no green, as they may only turn amber and then red when fire trucks, ambulances, or emergency vehicles of the like are exiting the station en route to an emergency. See also the "Unusual traffic-light usages" described below.

Actuated control

Some traffic lights at pedestrian crossings, especially those away from junctions, include a button which must be pressed in order to activate the timing system. This is generally accompanied by a large display reading "wait", which lights up when the button is pressed, and off when the lights enter the red phase. Often, other displays, such as countdowns or the green & red pedestrian lights are included in this panel. With the advent of computer-controlled traffic lights, many of these activation buttons have become obsolete. In fact, most in New York City have been disconnected.

Preemption

Main article: Traffic signal preemption

Traffic light in Munich, showing a special bus signal.

Emergency vehicles

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.

In one recent Oregon incident (2005) a fire engine pre-empted a signal at a light rail crossing, and proceeded to collide with a light-rail train. A subsequent inquiry determined that the light-rail driver was at fault, falsely believing that once the LRT had obtained the right-of-way across an intersection, it could not be lost until the train had cleared the intersection. Normally, this was the case, but pre-emption by an emergency vehicle was an exception to the rule.

Railroad pre-emption

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 (or flashing yellow) to allow traffic on the tracks to clear (in some cases, there are auxiliary traffic signals prior to the railroad crossing which will turn red, keeping new traffic from crossing the tracks. This is in addition to the flashing lights on the crossing gates themselves). After enough time to clear the crossing, the signal will turn. The crossing lights may begin flashing and the gates lower immediately, or this might be delayed until after the traffic light turns red.

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. Examples include the following:

• 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.
• The same thing is done to three traffic intersections on Telegraph Rd. between the Ohio state line and Monroe, Michigan. Two of these also include Right turn signals which are solid red when the Telegraph lights are blinking yellow.
• In Goshen, Indiana, the signals at the intersections on Lincolnway 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.
• At the intersection of Allen Rd. and Northline Rd. on the border of Southgate and Taylor, Michigan, a railroad track runs diagonally through the intersection from the northeast corner to the southwest. When the lights come on and the arm goes down, all lights turn red and two fiber-optic "no turn on red" signs light up facing Northline, since their right turners cross the tracks. A similar situation is found at the intersection of Oregon Route 10 (Farmington Road) and Lombard Street in Beaverton, Oregon.
• The Middle Tennessee Blvd./ Church St. intersection in Murfreesboro, Tennessee, gives Church Street (parallel to the tracks) a green with fiber-optic lit signs indicating "no right turn" or "no left turn" over the tracks, and Middle Tennessee Blvd. traffic faces a solid red light.

Unauthorised pre-emption

There have been some concerns that unauthorised people may have obtained devices that can trigger light preemption. The original 3M Opticom pre-emption system was activated by a 14 Hz strobe light added to the light bar of fire trucks, ambulances, and squad cars. When the sensor senses the 14 Hz strobe signal, the pre-emption is activated. The 14 Hz "secret" was eventually discovered, and MIRTs (Mobile InfraRed Transmitters) hit the market, consisting of a 14 Hz strobe with an infrared filter installed on it to make the light invisible to the naked eye. The use or sale of such devices in an unauthorised context was made illegal in the United States in 2005.

In some jurisdictions, traffic lights are set to turn red in all directions when the pre-emption system is activated, rather than holding one direction green to allow an emergency vehicle to proceed with traffic. This stops all traffic except for emergency vehicles, which are permitted to proceed through a red signal anyway, and thus removes much of the incentive for an unauthorised person to manipulate the pre-emption system to their own benefit. However, this can cause a disadvantage for the emergency vehicle because cars in front of it will stop at the intersection, blocking its path.

3M has developed an encrypted Opticom system. However, jurisdictions already using the original system would have to replace the original traffic signal sensors and vehicle-mounted emitters in order to use the encrypted system.

There have 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 unauthorised malicious person to abuse it as he sees fit. By issuing valid signal controlling messages from the area traffic control center if access is gained to it, 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 uses

Main article: Unusual uses 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.

Auto racing circuits can also use standard traffic signals to indicate to race 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 straightaways. 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.

Lane control

Lane control signals for countries driving on the right. Flashing red is sometimes used instead of yellow.

This parking ramp uses lane control signals so that the middle lane's direction may be changed according to time of day.

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 (see also reversible lane). Sometimes this is done as a way of managing rush hour traffic (one or more central lanes may flow inbound in the morning and outbound in the evening), in other cases the lanes are only reversed in unusual circumstances (such as a traffic accident or road construction closing one or more of the lanes). 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. (In the Australian state of Victoria, green and yellow are replaced by white, and there may be additional modes such as 'Centre lane turns only' at particular times of the day.) On Jarvis Street in Toronto, Edmonton and some other jurisdictions in Canada the lane control signals are employed without the use of an amber warning signal. Instead, the lane that is to undergo the direction reversal (the middle lane of a 5 lane downtown street) is marked with a red cross in both directions for a short period of time. This allows time for the lane to clear of traffic before a green arrow permits traffic in the reversed direction.

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 usually 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.

In the law

File:Redlightfinesign.jpg

Attempt to discourage red light running by posting the minimum fine.

File:Stoponredbanner.jpg

Another method is to simply remind drivers what red means through advertising campaigns.

In virtually all jurisdictions in which they are used, it is an offence for motorists (and cyclists) to disregard the instructions of traffic lights (or other traffic control devices). 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 criticised as ambiguous and difficult to enforce (red light cameras in Oregon are only activated if a motorist enters the intersection on a red). Red light cameras in NSW, Australia are only activated 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 he/she 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. 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.

A more unusual case of red light running is the Pittsburgh left, in which a driver waiting at a red light to turn left will immediately accelerate and turn left in front of 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 customary in the city of Pittsburgh.

By contrast, 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

Further information: Traffic enforcement camera

In some areas, a device usually called a red light camera has come into recent use. A 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.

Such cameras have evoked controversy on a number of fronts: in some jurisdictions, the fine cannot be contested, and is therefore seen by some as a violation of due process. Opposition has also stemmed from the practice of paying commissions to the companies which process the photographs from these cameras, as this is seen as an incentive to falsify images. Some have accused municipalities of purposely shortening the yellow-light intervals on intersections equipped with cameras in order to generate more fines. The presence of a red light camera is sometimes, but not always, indicated by a sign some distance before the intersection. Many red light cameras face the front of vehicles, thus it is possible for vehicles either registered in states that do not use or require front license plates (or vehicles illegally without front plates from states that do require them) to escape without being caught. Some further question whether red light cameras encourage safer behavior at intersections: since individuals may brake suddenly to avoid running a red light, rear end collisions increase when other drivers are not paying sufficient attention.

Repeated studies across US jurisdictions show appropriately-lengthened amber phases significantly reduce red-light violations, in both enforced and unpatrolled intersections.

Red light camera tickets have been contested on many of occasions by the fact that in many jurisdictions only a police officer is allowed to issue a ticket for a traffic violation and typically red light camera operations are contracted by a third party, not a policing agency. There have also been many cases where the owner was issued a ticket even though someone else may have been driving the vehicle and committed the violation, typically traffic laws refer to the driver who commits the violation and usually never places responsibility on the owner if they are not involved, such as in the case of a loaned vehicle, many courts however have not been willing to differ judgment to the driver rather than the owner in the case of red light cameras.

Houston, Texas has a unique system in which tickets for running the red light caught by the camera are a civil infraction rather than a criminal offense that appears on your driving record. If you were caught by a police officer, it would be a criminal offense that appears on your driving record. This is because the state has refused to endorse red light cameras. The same is true in Florida.

Light timing length

The length of yellow lights can differ state to state, for example in many states the length of an yellow light is usually 5 seconds, however in some cities or states the length of an yellow light may be as little as 2 or 3 seconds considerably reducing the time the driver has to react. It is typical for these times to be adjusted in line with the speed limit, faster speeds and a driver may be given more warning of a changing light. Also, the timing from when a red light is given and when a cross street is given a green light is usually based on the structure of the intersection its self, a delay of 2 seconds is typical, this allows time for a car passing through a intersection at the time a red light is given to clear the intersection. In a wider intersection, such as 4 lane road or highway intersection, this delay may be increased to as much as 4 seconds, again, allowing a driver who may have "run" the red light enough time to clear the intersection without causing an collision. It should be noted that not all traffic signals have a delay, with the conflicting traffic being given a green immediately after your light turns red. Many of these signals are seen in Texas, especially older signals.

In other contexts

The symbolism of a traffic light (and the meanings of the three primary colours used in traffic lights) are frequently found in many other contexts.

As a rating mechanism

In the British Civil Service and other government offices, traffic lights are used as a coding system for good or bad - usually known as a RAG rating. 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.

The traffic light coding system has also been introduced in the UK to rate packaged foods, with green indicating a healthy or recommended level of fat, salt or other nutrient, while amber and red show less healthy ratings.

In many factories, different stations on the production line(s) are equipped with factory monitoring and control systems; attached to such systems is a "traffic light" status indicator which is generally visible from many places within the factory. Green typically indicates normal levels of production; amber indicates that production has slowed (or attention is otherwise warranted); red indicates that production has stopped or the line is down.

Some organizations, such as the U.S. Navy, use traffic light terminology for sexual harassment education. Green light behavior is normal discussions or actions, such as discussing work or assisting someone on stairs. Yellow light behavior is potentially offensive to people, such as sexist jokes or patting someone on the behind. Red light behavior is obvious sexual harassment like sexual requests or stating a woman would get a promotion if she slept with the boss.

External links

United States

• Manual on Uniform Traffic Control Devices website (Federal Highway Administration)
• Animations of various US signal phasings
• mycrazyhobby.com - A crazy (but legal) street sign and traffic light collection.
• Traffic Lights by Ed Pegg, Jr., The Wolfram Demonstrations Project: demonstrates that there are "seven distinct light cycles that will prevent collisions."