Traffic light
A traffic light, also known as traffic signal, stop light, or semaphore, is a signaling device positioned at a road intersection, pedestrian crossing, or other location in order to indicate when it is safe to drive, ride, or walk using a universal color code.
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Introduction
Traffic lights for normal vehicles or pedestrians always have two main lights, a red light that means stop and a green (or sometimes white for pedestrians) light that means go. Usually, the red light contains some orange in its hue, and the green light contains some blue, to provide some support for people with red-green color blindness. In most countries there is also a yellow (or amber) light, which when on by itself and not flashing means stop if able to do so safely. In some systems, a flashing amber means that a motorist may go ahead with care if the road is clear, giving way to pedestrians and to other road vehicles that may have priority. A flashing red essentially means the same as a regular stop sign. There may be additional lights (usually a green arrow or "filter") to authorize turns. A turn light preceding the opposing through movement is called a leading left turn in the U.S., because it leads the opposing through green light (likewise, in the U.S., a left turn arrow that follows the opposing through movement is known as a "lagging left turn"). In Canada, a turn that is authorized before the opposing traffic, OR with an early left turn light is activated on the opposing side is called an Advanced Green. An advanced green can either mean that the traffic on your side could be going through and turning left before the opposing side sees a green. Or it could also mean both sides of traffic have an advanced turn green.
Traffic lights for special vehicles (such as buses or trams) may use other systems, such as vertical vs. horizontal bars of white light.
In most countries, the sequence is red (stop), green (go), amber (prepare to stop). In the UK and Canada, amber officially means 'stop' (unless it would cause an accident to do so)[1] but in practice, is treated as 'prepare to stop'. In the UK, Hong Kong, mainland China, Germany, Poland, Denmark and Iceland, among others, the sequence includes red and amber together before green, which helps draw attention to the impending change to green, to allow drivers to prepare to move off. (In many of these jurisdictions, such as the UK, 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/release the handbrake before the light turns green). The single flashing amber signal is used in the UK and Australia at Pelican crossings. Some older signals in New England, mainly near Boston, use the red + amber phase to signify an all-pedestrian phase ("Barnes dance"), as they were installed before pedestrian signals came to the area, and before the national standard prohibited them. See "Pedestrian scrambles" below, or the main article.
Depending on the jurisdiction, traffic may turn after stopping on a red (right in right-driving countries; left in left-driving countries), provided they yield to pedestrians and other vehicles. In some jurisdictions which generally forbid this, a green arrow sign next to the traffic light indicates that it is allowed at a particular intersection. Conversely, jurisdictions which generally allow this might forbid it at a particular intersection with a "no turn on red" sign, or might put a green arrow to indicate specifically when a right turn is allowed without having to yield to pedestrians (this is usually when traffic from the perpendicular street is making a left turn onto one's street and thus no pedestrians are allowed in the intersection anyway). Some jurisdictions allow turning on red in the opposite direction (left in right-driving countries; right in left-driving countries) from a one-way road onto another one-way road; some of these even allow these turns from a two-way road onto a one-way road. Also differing is whether a red arrow prohibits turns; some jurisdictions require a "no turn on red" sign in these cases. A study in the State of Illinois concluded that allowing drivers to proceed straight on red after stopping, at specially posted T-intersections where the intersecting road 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. [2]
In France, Spain, and Romania permission to turn right (or more rarely to turn left or to go straight on) on a red light is indicated by a flashing amber arrow (cars do not have to stop but must yield way to other cars and pedestrians).
Another distinction is between intersections that have dedicated signals for turning across the flow of opposing traffic and those that do not. Such signals are called dedicated left-turn lights in the United States (since opposing traffic is on the left). 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). A third type is known as a "doghouse" or "cluster head" - a vertical column with the two normal lights is on the right hand 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. If there is no left-turn signal, one must yield to opposing traffic and turn when it is safe to do so. In the U.S., many inner-city and rural areas do not have such dedicated lights, while most suburban areas have them. Such lights tend to make intersections safer by reducing the risk of head-on collisions and may speed up through traffic, but may decrease the overall efficiency of the intersection as it becomes congested, depending on what proportion of traffic is turning.
In the UK, traffic lights only applicable to traffic going a certain direction are called "filter lights" and are indicated as a green arrow pointing in the direction traffic is allowed to proceed. Thus, traffic lights displaying a red light and a green arrow to the left allow traffic turning left to proceed, but all other traffic must remain stopped. This is commonly used at junctions where traffic needs to turn across oncoming traffic. Traffic turning right can proceed on a full green if safe to do so, but if a filter right light is display this indicates that oncoming traffic has been stopped.
Traffic light failure in most jurisdictions must be handled by drivers as a priority-to-the-right intersection in Europe, or a four-way stop elsewhere, pending the arrival of a police officer to direct traffic.
In some countries, pedestrian traffic lights include a type of siren or warbler, which sounds during the red phase, in order to alert visually impaired pedestrians that it is safe to cross. These are generally 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, generally in the form of a cone, attached to the base of the 'wait' button panel, which rotates during the red phase, to help deafblind people cross the road.
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.
Some jurisdictions, such as Alberta, Saskatchewan, New Mexico, Quebec, Texas, Wisconsin, and Yukon Territory (specifically Whitehorse), 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.[3] 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.
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 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 fresnel example common in the USA is known as a 3M "Program Head", although 3M has recently discontinued the line.
Shades and backpanels 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 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
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.
On Potsdamer Platz, Berlin Germany, it is widely claimed (though this is subject to some disagreement), that the world's first electric street lights were installed there in 1882. What is not refuted is that Europe's first traffic lights were erected there in 1924 in an attempt to control the sheer volume of traffic passing through. These lights were mounted on a five-sided 8.5 metre high tower, at the top of which a policeman sat in a small cabin and switched the lights manually, though they were automated after a few years (a replica of this tower was erected in the late 1990s close to its original location).
The modern electric traffic light is an American invention.[4] As early as 1912 in Salt Lake City, Utah, policeman Lester Wire set up the first red-green electric traffic lights. On 5 August 1914, the American Traffic Signal Company installed a traffic signal system on the corner of 105th Street and Euclid Avenue in Cleveland, Ohio. Based on the design of James Hoge, it had two colors, red and green, and a buzzer to provide a warning for color changes. The first four-way, three-color traffic light was created by police officer William Potts in Detroit in 1920.[1] In 1923, Garrett Morgan patented a traffic signal device, although it did not directly impact the evolution of the modern traffic light [5].
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 [6]. The first automatic experimental traffic lights in England were deployed in Wolverhampton in 1927. [7]
The first automated traffic light was introduced in Toronto, Ontario, Canada in the 1950s.[citation needed]Ampelmännchen pedestrian traffic signals have come to be seen as a nostalgic sign for the former German Democratic Republic.
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 have numerous advantages over incandescent lamps; among them are:
- Much greater energy efficiency
- 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 even in direct sunlight.
- 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 a sudden burn-out like incandescent-based lights, LEDs start to gradually dim out when they're past their prime, allowing transportation maintenance departments to be warned well in advance as to when to change the light.
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. 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.
In some areas, LED-based signals have been fitted (or retrofitted) with special Fresnel 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.
Controller system
In the late 1990s, a national standardization 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 standardization 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.
Since the 1980s, some traffic signals have switched to computer-based controllers.
Pedestrian scrambles
A pedestrian scramble, or Barnes Dance (named for Henry Barnes), is a special traffic light that stops all vehicular traffic. Pedestrians then have exclusive access to the intersection and can diagonally cross the intersection. Pedestrian scrambles are useful when there is heavy diagonal pedestrian traffic, or heavy pedestrian traffic in general. In intersections with heavy pedestrian traffic, pedestrians have the right-of-way, blocking drivers from turning. A pedestrian scramble gives vehicles exclusive access to the intersection for a period of time as well.
Usually these are displayed as simply a red signal in all directions with walk signals; in some areas such as in the Boston area, 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.
More sophisticated control systems use electronic sensor loops 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 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). 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 synchronized signals is very important. Synchronized 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 signalized intersection spacing and expected vehicle speeds. In some countries (e.g. Germany 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, or "groene golf" in Dutch (English: "green wave").
In modern coordinated signal systems (such as US 24, Telegraph Rd, and M-3, Gratiot Avenue, in suburban Detroit, Michigan, USA, or former Highway 8 in downtown Hamilton, Ontario, Canada), it is possible for drivers to go many miles 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 utilize the green indication. In synchronized systems, however, drivers will often use excessive speed in order to "make" as many lights as possible.
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. 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.
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.
Preemption
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.[2] 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 cyle 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.
- 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.
Unauthorized pre-emption
There have been some concerns that unauthorized 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 unauthorized context was made illegal in the United States in 2005.[3]
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 unauthorized person to manipulate the pre-emption system to their own benefit.
3M has developed an encrypted Opticom system.[4] 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 unauthorized 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.
Other attempts to violate preemptive traffic light systems are being created. "Foxmanifestj" and others from Jinx Hacking Forums believe it is possible to create a .wav file that, with the aid of an Audio-to-Infrared converter, will easily change traffic lights from any vehicle using an MP3 player.
Unusual traffic-light uses
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
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, 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 always placed horizontally. Signals that may indicate other conditions for roads without counterflow lanes also exist, such as those that indicate different speed limits for different lanes.
Traffic lights and the law
In virtually all jurisdictions in which they are used, it is a legal offense for motorists 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 a yellow 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 a yellow 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 only activated if a motorist enters the intersection on a red).
In some jurisdictions (such as San Francisco and New York City), 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 left turn on a red light at a busy intersection, by pulling partway into the intersection at a green light waiting to turn left, and, if oncoming traffic is not abated before the light changes to red, proceeding to turn left once the light has turned red and opposing traffic has stopped.
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.
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, 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 being caught.
Traffic lights 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.
Use as a rating mechanism or an indication of status
In the UK's 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.
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 butt. Red light behavior is obvious sexual harassment like sexual requests or stating a woman would get a promotion if she slept with the boss.
Cellular Automata
In Conway's game of life, a traffic light pattern consists of four "blinkers" in a cross-shaped configuration.
Humour
A member of the humorous British website B3ta, known as "Koit", created a series of GIF animations exploring the (often explosive) relationship between the red and green men featured on a UK pedestrian crossing sign. These animations are now featured on their own website, Traffic Light Wars.
Notoriously silly British comedy troupe Monty Python paid homage to the red, amber, and green in their song "I Like Traffic Lights." The drab and near monotone song is included in their 1988 compilation album, The Final Rip Off.
In the premiere episode of Sliders, Quinn Mallory briefly visits a parallel Earth where the colors were used in reverse meaning: red (at top) meant go, green (at bottom) meant stop. Of course, other aspects of that Earth were reversed: Elvis Presley was alive, and CDs were obsolete, being replaced by vinyl records.
The movie Starman conjectured that an alien might come to the wrong conclusion through observation of how traffic lights function:
- "I watched you very carefully. Red light stop, green light go, yellow light go very fast."
References
- ^ Moyer, Sheldon (March 1947). Mr. 'Trafficlight'. Motor News: 14-15,27.
- ^ Emergency vehicle traffic signal preemption system. United States Patent and Trademark Office. Retrieved on October 7, 2005.
- ^ Public Law 109-59, Section 2018
- ^ 3M Opticon Signal Pre-Emption System
External links
- Manual on Uniform Traffic Control Devices website (Federal Highway Administration)
- Inventing history: Garrett Morgan and the traffic signal
- Examples of traffic lights (animated GIFs)
- Traffic System Security concerns, News article
- Flaws in traffic system security outlined
- How does a traffic light detect a car?
- Signalfan
- Traffic Signal Museum
- Animations of various US signal phasings
- MyCrazyHobby - A Private Collection of antique Traffic Lights and Road Signs
Categories
Articles with unsourced statements | Pedestrian crossing components | Road transport | Traffic law | Traffic signs