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Traffic light
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Traffic lights, also known as traffic signals, stop lights, traffic lamps, stop-and-go lights, robots or semaphore, are signaling devices positioned at road intersections, pedestrian crossing, or other location to control the flow of traffic.
Traffic lights have been installed in most cities around the world to control the flow of traffic, and thereby to reduce the risk of accidents.
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Encyclopedia
Traffic lights, also known as traffic signals, stop lights, traffic lamps, stop-and-go lights, robots or semaphore, are signaling devices positioned at road intersections, pedestrian crossing, or other location to control the flow of traffic.
Traffic lights have been installed in most cities around the world to control the flow of traffic, and thereby to reduce the risk of accidents. They assign the right of way to road users by the use of lights in standard colors (Red - Amber - Green), using a universal color code (and a precise sequence, for those who are color blind). They are used at busy intersections to more evenly apportion delay to the various users. They reduce delay for side road traffic and pedestrians, which otherwise may have to wait an unacceptable length of time for a gap in traffic.
The most common traffic lights consist of a set of three lights: red, yellow (officially amber), and green. When illuminated, the red light indicates for vehicles facing the light to stop; the amber indicates caution, either because lights are about to turn green or are about to turn red; and the green light to proceed past the light (with care is always implied).
There are many variations in traffic lights, depending on the customs of a country and the special needs of a particular intersection. There may, for example, be special lights for pedestrians, bicycles, buses, trams, etc. There may also be separate sets of lights for turning traffic. All of these, and others, may be used at the one intersection.
Signals can have a beneficial effect of traffic safety. By separating conflicting streams of traffic in time, the chances of right-angle collisions are reduced. However, there is sometimes a tradeoff, since rear-end crashes often increase after a signal is installed. Since rear-end crashes are usually less severe than right-angle crashes, the net result is often, but not always, beneficial.
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.
The modern electric traffic light is an American invention. As early as 1912 in Salt Lake City, Utah, policeman Lester Wire invented the first red-green electric traffic lights. On 5 August 1914, the American Traffic Signal Company installed a traffic signal system on the corner of East 105th Street and Euclid Avenue in Cleveland, Ohio. It had two colors, red and green, and a buzzer, based on the design of James Hoge, to provide a warning for color changes. The design by James Hoge allowed police and fire stations to control the signals in case of emergency. The first four-way, three-colour traffic light was created by police officer William Potts in Detroit, Michigan in 1920. In 1923, Garrett Morgan patented a traffic signal device. It was Morgan's experience while driving along the streets of Cleveland that led to his invention of a traffic signal device. Ashville, Ohio claims to be the location of the oldest working traffic light in the United States, used at an intersection of public roads until 1982 when it was moved to a local museum.
The first interconnected traffic signal system was installed in Salt Lake City in 1917, with six connected intersections controlled simultaneously from a manual switch. Automatic control of interconnected traffic lights was introduced March 1922 in Houston, Texas. The first automatic experimental traffic lights in England were deployed in Wolverhampton in 1927.
Ampelmännchen pedestrian traffic signals have come to be seen as a nostalgic sign for the former German Democratic Republic.
The colour 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. 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 installed a traffic light with green on the top, because 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.
Road rules
Traffic lights have been installed in most cities around the world to control the flow of traffic at intersections, and thereby to reduce the risk of accidents. Traffic lights are an integral part of the road rules for traffic that apply in any situation. Road rules vary around the world, as do the meaning of various traffic lights.
There are many variations in traffic lights, depending on the customs of a country and the special needs of a particular intersection. There may, for example, be special lights for pedestrians, bicycles, buses, trams, etc. There may also be separate sets of lights for turning traffic. All of these, and others, may be used at the one intersection.
Types of traffic lights
Three-set lights
The most common traffic lights consist of a set of three lights: red, amber (sometimes called yellow) and green. When illuminated, the red light indicates for vehicles facing the light to stop; the amber indicates caution, either because lights are about to turn green or are about to turn red; and the green light to proceed past the light (with care is always implied).
The universal standard is for the red to be above the green, and if there is also an amber it is placed in the middle. If the three-set lights are mounted horizontally, the red will typically be to the left of the green. The standards apply whether the country drives on the left or the right.
Each country has differing road rules, including how traffic lights are to be interpreted. For example, in some countries, a flashing yellow light means that a motorist may proceed with care if the road is clear, giving way to pedestrians and to other road vehicles that may have priority (essentially the same as arriving at a non-signalized intersection and not facing a stop sign). A flashing red may be treated as a regular stop sign.
In most countries, the sequence is green (go), amber (prepare to stop), and red (stop). In some systems, however, just before 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 is intended to give the driver time to select first gear or release the handbrake before the light turns green, but in practice is treated as an invitation to go before the green light is showing. Austrian, Israeli, and Mexican traffic lights use a flashing green 'prepare to stop' prior to the amber 'stop if safe' signal.
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, Serbia, Austria, Israel, and parts of Canada and Mexico, 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. It is used in Serbia and the United States to mark places where greater attention is needed (dangerous crossings, sharp curves etc.). In Canada, a flashing amber light means "drive with caution" and is frequently combined with a flashing red light (meaning "stop") at four-way intersections. In many S.E, Asian countries (e.g. Thailand) a flashing amber light indicates a driver may proceed cautiously across a junction where signals only operate at busy periods.
Pedestrian crossing lights
Traffic lights for pedestrians normally have two main lights: a red light that means 'stop' and a green light that means 'go' (or, more correctly, 'proceed with caution'). There is usually a flashing phase (red in the US and Australia, green in Europe) that means 'complete your crossing'. In most locales in North America, the colors used are a red-orange ("Portland orange") for "stop/wait" and a bluish-white ("lunar") for "go." While the "walk" signal is generally a walking human figure, North American pedestrian signals usually show an upraised hand for "stop," while most other countries display a standing human figure. Some older American signals display the verbal commands "Walk" (lunar white or green) and "Don't Walk" or "Wait" (red-orange).
At selected pedestrian crossings in some countries, pedestrian traffic lights include a type of siren, beeper or warbler, which sounds in order to alert visually impaired pedestrians that it is safe to cross. 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.
Some pedestrian crossing lights are only activated after a pedestrian presses an activating button, while others operate automatically, and others still operate automatically but only at certain times of the day.
Special lights
Traffic lights for special vehicles (such as buses) may use other systems, such as vertical or horizontal bars of a white light. In Portland, Oregon, the tram signals feature a horizontal white bar and an orange vertical bar. Some systems use the letter B for buses, and T for trams. There are also signs of a bicycle for cyclists.
Colors
The most common colors used in traffic lights are red, yellow (officially amber), and green. Red typically means stop or high level of danger; yellow typically means caution; and green typically means proceed with care.
The use of these colors is 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 color blindness. In the UK, traffic lights typically have a white reflective border which enables color blind users, during the hours of darkness, to distinguish the lights from other similarly-colored street or automobile lights, and to allow them to distinguish the lights by vertical position (top/middle/bottom; British traffic lights are never mounted horizontally). Although the word "yellow" is common used in the United States, the color usually is only slightly more yellowish than the amber used in Europe.
Turning signals and rules
In some instances, traffic may turn left after stopping on a red (right in right-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.. 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. Cluster signals in Australia and New Zealand use six signals, the sixth being a red arrow which can operate separately from the standard red light. In a fourth type, sometimes seen at intersections in Ontario and Quebec, Canada, there is no dedicated left-turn lamp per se. Instead, the normal green lamp flashes rapidly, indicating permission to go straight as well as make a left turn in front of opposing traffic, which is being held by a steady red lamp. (This "flashing green" can be somewhat startling and confusing to drivers not familiar with this system. This also can cause confusion amongst visitors to British Columbia, where a flashing green signal denotes a pedestrian controlled intersection.) Another interesting practice seen at least in Ontario is that cars wishing to turn left that arrived after the left turn signal ended can do so during the amber phase, as long as there is enough time to make a safe turn.
A flashing amber arrow, which allows drivers to make left turns after giving way to oncoming traffic, is becoming more widespread in the United States, particularly in Oregon. In the normal sequence, a protected green left-turn arrow will first change to a solid amber arrow to indicate the end of the protected phase, then to a flashing amber arrow, which remains flashing until the standard green light changes to amber and red. In Oregon, the amber flashing arrow is usually housed in a separate light head from the steady amber arrow, in order to provide a visible position change. These generally take the form of four signal heads (green, amber, amber, red). On some newer signals, notably in the city of Bend, the green and flashing amber arrows emanate from the same light head through the use of a dual-color LED array, while the solid amber arrow is mounted above it.
Generally, a dedicated left-turn signal is illuminated at the beginning of the green phase of the green-yellow-red-green cycle. This 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.
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.
At intersections where no turns are allowed from any direction, the green light can be replaced with a green arrow pointing up.
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 as well as some jurisdictions in the US, traffic lights inactive at nighttime emit an amber-colored flashing signal in directions owing priority while the intersecting street emit a flashing red light, requiring drivers to stop before proceeding.) In the UK and parts of North America, drivers simply treat the junction as being uncontrolled when traffic lights fail, giving way as appropriate, unless a police officer is present. In much of the United States failed traffic signals must be treated as all-way stop interesections. 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 .
Mounting
There are significant differences from place to place in how traffic lights are mounted or positioned so that they are visible to drivers. Depending upon the location, traffic lights may be mounted on poles situated on street corners, hung from horizontal poles or wires strung over the roadway, or installed within large horizontal gantries that extend out from the corner and over the right-of-way. In the last case, such poles or gantries often have a lit sign with the name of the cross-street.
Japan, South Korea, Taiwan, Thailand, and Croatia, along with some jurisdictions in the U.S. (including all of the State of New Mexico), Canada, and Mexico mount lights with their multiple faces arranged horizontally, often with supplemental vertical signals on the side, while others use vertical signals almost exclusively. Horizontal signals have consistent orientation, like their vertical counterparts. Often, supplemental curb pedestal mounts, intended to support a signal for a different approach road, are used when primary signals are partially obscured due to structures such as overpasses, approaches around a building that obscures the primary signal mountings, and unusual approach geometry. In Florida, horizontal signals mounted on poles, known as "mast arms", are in wide use due to their lower wind profile, important for minimising hurricane damage.
California is particularly rigorous in ensuring that drivers can see the current state of a traffic light. One entrance to a typical large intersection, with three through lanes, two dedicated left-turn lanes, and a crosswalk, may have as many as three traffic lights for the left-turn lanes, three for the through lanes, and a pedestrian signal for the crosswalk. Those numbers must be multiplied by four to cover all four ways to enter a typical intersection.
In addition to being positioned and mounted for desired visibility for their respective traffic, some traffic lights are also aimed, louvered, or shaded to minimize mis-interpretation from other lanes. For example, a Fresnel lens on an adjacent through-lane signal may be aimed to prevent left-turning traffic from anticipating its own green arrow. One example of the fresnel lens application common in the USA is the 3M Model 131 traffic signal (dubbed the "Programmed Visibility" signal). Although 3M has recently discontinued the M-131, it became a popular traffic signal for skewed or complex intersections. Today, McCain is the only U.S.-based manufacturer producing "programmable" traffic signals. In addition to aiming, fresnel lenses, and louvers, visors and back panels are also useful in areas where sunlight would diminish the contrast and visibility of a signal face.
Traffic signals in most areas of Europe are located at the stop line on same side of the intersection as the approaching traffic 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.
Technology
Optics and lighting
In the mid 1990s, cost-effective traffic light lamps using light-emitting diodes (LEDs) were developed; prior to this date traffic lights were designed using incandescent or halogen light bulbs. Unlike the incandescent-based lamps, which use a single large bulb, the LED-based lamps consist of an array of LED elements, arranged in various patterns. When viewed from a distance, the array appears as a continuous light source.
LED-based lamps (or 'lenses') have numerous advantages over incandescent lamps; among them are:
- Much greater energy efficiency (can be solar-powered).
- Much longer lifetime between replacement, measured in years rather than months. Part of the longer lifetime is due to the fact that some light is still displayed even if some of the LEDs in the array are dead.
- Brighter illumination with better contrast against direct sunlight, also called 'phantom light'.
- The ability to display multiple colors and patterns from the same lamp. Individual LED elements can be enabled or disabled, and different color LEDs can be mixed in the same lamp
- Much faster switching.
- Instead of sudden burn-out like incandescent-based lights, LEDs start to gradually dim when they wear out, warning transportation maintenance departments well in advance as to when to change the light. Occasionally, particularly in green LED units, segments prone to failure will flicker rapidly beforehand.
The operational expenses of LED-based signals are far lower than equivalent incandescent-based lights. As a result, most new traffic light deployments in the United States, Canada and elsewhere have been implemented using LED-based lamps; in addition many existing deployments of incandescent traffic lights are being replaced. In 2006, Edmonton, Alberta, Canada completed a total refit to LED-based lamps in the city's over 12,000 intersections and all pedestrian crosswalks. Many of the more exotic traffic signals discussed on this page would not be possible to construct without using LED technology. However, color-changing LEDs are in their infancy and may surpass the multi-color array technology.
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.
Another new LED technology is the use of CLS (Central Light Source) optics. These comprise around 7 high-output LEDs (sometimes 1 watt) at the rear of the lens, with a diffuser to even out and enlarge the light. This gives a uniform appearance, more like traditional halogen or incandescent luminaries.
Replacing halogen or incandescent reflector and bulb assemblies behind the lens with an LED array can give the same effect. This also has its benefits: minimal disruption, minimal work, minimal cost and the reduced need to replace the entire signal head (housing).
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 are standard in most jurisdictions in Ontario, Manitoba, and British Columbia.
In the United Kingdom, 300mm (or 12 inch) optics were only implemented with Mellor Design Signal heads designed by David Mellor. These were designed for symbolic optics only to compensate for the light loss caused by the symbol. With the invention of anti-phantom, highly visible SIRA lenses, 200mm (8 inch) symbolic aspects could be designed to give the same light output as plain lenses, so a larger surface area was not needed. Consequently 300mm lenses are no longer approved for use in the UK and all lenses installed on new installations have to be 200mm in accordance with TSRGD (Traffic Signs Regulations and General Directions). Exemptions to this rule include temporary or replacement signals.
Control and coordination
The normal function of traffic lights requires sophisticated control and coordination to ensure that traffic moves as smoothly and safely as possible and that pedestrians are protected when they cross the roads.
Pedestrian indications
In China, Singapore, Malaysia, Vietnam, Taiwan, North America and Europe, 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). Pedestrian countdown signals are also becoming more common in the United States.
A pedestrian scramble, scramble corner, 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.
Preemption
Some regions have signals that are interruptible, giving priority to special traffic. Such traffic light preemption is usually reserved for emergency vehicles such as fire apparatus, ambulances, and police squad cars, though sometimes mass transit vehicles including buses and light rail trains can interrupt lights. Most of the systems operate with small transmitters that send radio waves, infrared signals, or strobe light signals that are received by a sensor on or near the traffic lights. Some systems use audio detection, where a certain type of siren must be used and detected by a receiver on the traffic light structure.
Upon activation the normal traffic light cycle is suspended and replaced by the "preemption sequence": the traffic lights to all approaches to the intersection are switched to "red" with the exception of the light for the vehicle that has triggered the preemption sequence. Sometimes, an additional signal light is placed nearby to indicate to the preempting vehicle that the preempting sequence has been activated and to warn other motorists of the approach of an emergency vehicle. The normal traffic light cycle resumes after the sensor has been passed by the vehicle that triggered the preemption.
In lieu of pre-emptive mechanisms, in most jurisdictions, emergency vehicles are not required to respect traffic lights, but must activate their own emergency lights when crossing an intersection against the light, in order to alert oncoming drivers to the preemption.
Variations in traffic light use
In many regions, traffic lights function differently or have different displays depending on available technology, traffic patterns, or other vehicles such as trolleys that also use the intersection. For example, some fixtures feature a flashing green light or more than one arrow lit at one time. An example of a flashing green light found in Canada, to notify left turning drivers that they have the right of way and that the opposing lanes will not be moving.
Racing
Automobile racing circuits can also use standard traffic signals to indicate to racing car drivers the status of racing. On an oval track, four sets may be used, two facing a straight-away and two facing the middle of the 180 degree turn between straight-aways. Green would indicate racing is under way, while yellow would indicate to slow or while following a pace car; red would indicate to stop, probably for emergency reasons.
Scuderia Ferrari, a Formula 1 racing team, formerly used a traffic light system during their pit stops to signal to their drivers to when to leave the pits. The red light was on when the tyres were being changed and fuel was being added, yellow was on when the tyres were changed, and green was on when all work was completed. The system is (usually) completely automatic. However, the system was withdrawn after the 2008 Singapore Grand Prix, due to the fact that it heavily delayed Felipe Massa during the race, when he was in the lead. Usually, the system was automatic, but heavy traffic in the pit lane forced the team to operate it manually. A mechanic accidentally pressed the green light button when the fuel hose was still attached to the car, causing Massa to drive off, towing the fuel hose along. Additionally, Massa drove into the path of Adrian Sutil, earning him a penalty. He finally stopped at the end of the pit lane, forcing Ferrari's mechanics to sprint down the whole of the pit lane to remove the hose. As a result of this, and the penalty he also incured, Massa finished 13th. Ferrari decided to use a traditional "lollipop" for the remainder of the 2008 season.
Dummy lights
A dummy light is a traffic light which stands on a pedestal in the middle of an intersection, hence the use of the term "dummy." There are at least three which still operate in the United States today, all located in New York State: Beacon, Canajoharie and Croton-on-Hudson. There have been number of requests in recent years for these traffic lights to be removed due to safety concerns, but the historic value have kept these landmarks at their original locations.
Lane control
Lane control lights are a specific type of traffic light used to manage traffic on a multi-way road or highway. Typically they allow or forbid traffic to use or more of the available lanes by the use of green lights or arrows (to permit) or by red lights or crosses (to prohibit).
Legal implications
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 criticized as ambiguous and difficult to enforce (red light cameras in Oregon are activated only if a motorist enters the intersection on a red). Red light cameras in NSW, Australia are activated only if a motorist enters an intersection 0.3 seconds after the light has turned red.
In some jurisdictions (such as Toronto, Washington D.C., New York City), and the state of California, there are ordinances against "gridlocking--any motorist who enters an intersection (even if on a green light) but does not ensure that one can proceed through the intersection and gets stuck in the middle of the intersection (when traffic ahead fails to proceed), and remains there after the light turns red (thus blocking traffic coming from other directions) may receive a citation. This is sometimes used as a justification for making a turn across the opposing travel lanes on a red light at a busy intersection, by pulling partway into the intersection at a green light waiting to perform the turn, and, if oncoming traffic is not abated before the light changes to red, proceeding to turn once the light has turned red and opposing traffic has stopped. This means that at busy junctions without a protected green arrow for turning traffic, one turns after the light turns red. This maneuver is commonly referred to as "occupying the intersection" or "being legally allowed to complete one's turn". In some jurisdictions, including most American states, a vehicle already in the intersection when the light turns red legally has the right of way, and vehicles who have green must yield to the vehicle in the intersection.
Some local driving traditions may be legally questionable. A prime example is the Pittsburgh left. Although failing to yield to oncoming traffic while navigating a turn is a serious traffic violation and is prohibited in the Commonwealth of Pennsylvania, many drivers will make a Pittsburgh left. The Pittsburgh Left: a driver waiting at a red light to turn left is allowed to turn by oncoming traffic when the light turns green, instead of yielding the right-of-way as the law requires. Although illegal, such behavior is regarded as not only customary but also polite in the city of Pittsburgh.
Similarly, in Sackville, New Brunswick and most of eastern Massachusetts it is customary for through traffic to voluntarily yield to the first oncoming left-turning vehicle to allow it to perform a Pittsburgh left. This is similar to a hook turn performed in Melbourne, Australia which is legal at signed intersections.
Enforcement of traffic lights is done in one of several ways:
- By police officers observing traffic through the light, and pulling over and issuing citations to motorists who violate the signal.
- As a result of an accident investigation, if it is determined that one or more motorists ran the red light — even if the incident was not observed by a police officer.
- With red light cameras.
Red light cameras
In some areas, red light cameras are used. An automated camera is connected to the triggering mechanism for the corresponding traffic light, which is targeted to photograph any vehicle which crosses against the light. The driver or owner (depending on local laws) of a vehicle so photographed can then be fined for violating traffic laws.
Light timing length
The length of 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 a 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 colors used in traffic lights) are frequently found in many other contexts. Since they are often used as single spots of color without the context of vertical position, they are typically not comprehensible to up to one in ten males who are color blind.
As a rating mechanism
see also Traffic light rating system
The colours red, amber and green are commonly used as a simple to understand rating system in a variety of business and manufacturing situations.
See also
External links
United States
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