Traffic light
Overview
 
Traffic lights, which may also be known as stoplights, traffic lamps, traffic signals, signal lights, robots or semaphore, are signalling devices positioned at road intersections, pedestrian crossing
Pedestrian crossing
A pedestrian crossing or crosswalk is a designated point on a road at which some means are employed to assist pedestrians wishing to cross. They are designed to keep pedestrians together where they can be seen by motorists, and where they can cross most safely across the flow of vehicular traffic...

s and other locations to control competing flows of traffic. Traffic lights were first installed in 1868 in London, and today are installed in most cities around the world.
Encyclopedia
Traffic lights, which may also be known as stoplights, traffic lamps, traffic signals, signal lights, robots or semaphore, are signalling devices positioned at road intersections, pedestrian crossing
Pedestrian crossing
A pedestrian crossing or crosswalk is a designated point on a road at which some means are employed to assist pedestrians wishing to cross. They are designed to keep pedestrians together where they can be seen by motorists, and where they can cross most safely across the flow of vehicular traffic...

s and other locations to control competing flows of traffic. Traffic lights were first installed in 1868 in London, and today are installed in most cities around the world. Traffic lights alternate the right of way of road users by displaying lights of a standard colour (red, yellow/amber, and green), using a universal color code
Color code
A color code is a system for displaying information by using different colors. Color codes are often difficult for color blind and blind people to interpret....

 (and a precise sequence to enable comprehension by those who are color blind
Color blindness
Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...

).

In the typical sequence of coloured lights:
  • Illumination of the green light allows traffic to proceed in the direction denoted,
  • Illumination of the orange/yellow light denoting, if safe to do so, prepare to stop short of the intersection, and
  • Illumination of the red signal prohibits any traffic from proceeding.


Usually, the red light contains some orange in its hue
Hue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...

, and the green light contains some blue, for the benefit of people with red-green color blindness, and "green" lights in many areas are in fact blue lenses on a yellow light (which together appear green).

History

On December 10, 1868, the first traffic lights were installed outside the British Houses of Parliament
Palace of Westminster
The Palace of Westminster, also known as the Houses of Parliament or Westminster Palace, is the meeting place of the two houses of the Parliament of the United Kingdom—the House of Lords and the House of Commons...

 in London
London
London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

, by the railway engineer J. P. Knight
J. P. Knight
John Peake Knight, a railway engineer, invented the original traffic light in 1868. This was not the traffic light we know now, but was a revolving gas-powered lantern with a red and a green light. Knight's invention was similar to the railway signals of the time...

. They resembled railway signal
Railway signal
A signal is a mechanical or electrical device erected beside a railway line to pass information relating to the state of the line ahead to train/engine drivers. The driver interprets the signal's indication and acts accordingly...

s of the time, with semaphore
Railway semaphore signal
One of the earliest forms of fixed railway signal is the semaphore. These signals display their different indications to train drivers by changing the angle of inclination of a pivoted 'arm'. Semaphore signals were patented in the early 1840s by Joseph James Stevens, and soon became the most...

 arms and red and green gas lamps for night use. The gas lantern was turned with a lever at its base so that the appropriate light faced traffic. Unfortunately, it exploded on 2 January 1869, injuring or killing the policeman who was operating it. Herbet Thorp wired the first permanent traffic lights in England. They are in Leeds.

The modern electric traffic light is an American invention
Invention
An invention is a novel composition, device, or process. An invention may be derived from a pre-existing model or idea, or it could be independently conceived, in which case it may be a radical breakthrough. In addition, there is cultural invention, which is an innovative set of useful social...

. As early as 1912 in Salt Lake City, Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...

, policeman Lester Wire
Lester Wire
Lester Farnsworth Wire is credited with the invention of the traffic light in 1912 in Salt Lake City. Wire worked as a detective for the Salt Lake City police force.He was buried at Salt Lake City Cemetery....

 invented the first red-green electric traffic lights. On August 5, 1914, the American Traffic Signal Company installed a traffic signal system on the corner of East 105th Street and Euclid Avenue in Cleveland, Ohio
Ohio
Ohio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...

. It had two colors, red and green, and a buzzer
Buzzer
A buzzer or beeper is an audio signaling device, which may be mechanical, electromechanical, or piezoelectric. Typical uses of buzzers and beepers include alarm devices, timers and confirmation of user input such as a mouse click or keystroke....

, 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
Traffic signal preemption
Traffic signal preemption is a type of system that allows the normal operation of traffic lights to be preempted, often to assist emergency vehicles...

. The first four-way, three-color traffic light was created by police officer William Potts
William Potts
William Potts , a Detroit police officer, is credited with inventing the modern, three-way traffic light in Detroit in 1920 ....

 in Detroit, Michigan
Michigan
Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....

 in 1920. In 1922, T.E. Hayes patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....

ed his "Combination traffic guide and traffic regulating signal" (Patent # 1447659). Ashville, Ohio
Ashville, Ohio
Ashville is a village in Pickaway County, Ohio, United States. The population was 3,174 at the 2000 census.-History:A historical museum, Ohio's Small Town Museum, is operated in the community. The museum claims to be home to America's oldest working traffic light, which directed traffic in...

 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
Automatic
- Technology :* Automatic transmission, or a car with an automatic transmission* Automatic firearm* Automatic watch* Automatic , a defunct American automobile company- Albums :* Automatic * Automatic...

 control of interconnected traffic lights was introduced March 1922 in Houston, Texas
Houston, Texas
Houston is the fourth-largest city in the United States, and the largest city in the state of Texas. According to the 2010 U.S. Census, the city had a population of 2.1 million people within an area of . Houston is the seat of Harris County and the economic center of , which is the ...

. The first automatic
Automatic
- Technology :* Automatic transmission, or a car with an automatic transmission* Automatic firearm* Automatic watch* Automatic , a defunct American automobile company- Albums :* Automatic * Automatic...

 experimental traffic lights in England were deployed in Wolverhampton
Wolverhampton
Wolverhampton is a city and metropolitan borough in the West Midlands, England. For Eurostat purposes Walsall and Wolverhampton is a NUTS 3 region and is one of five boroughs or unitary districts that comprise the "West Midlands" NUTS 2 region...

 in 1927.
In 1923, Garrett Morgan patented his own version. The Morgan traffic signal was a T-shaped pole unit that featured three hand-cranked positions: Stop, in all -directional stop position. This third position halted traffic in all directions to allow pedestrians to cross streets more safely. Its one "advantage" over others of its type was the ability to operate it from a distance using a mechanical linkage.
Toronto was the first city to computerize its entire traffic signal system, which it accomplished in 1963.

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

Timer
Timer
A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...

s on traffic lights were introduced in the 1990s. Though uncommon in most American urban areas, timers are still used in some other Western Hemisphere countries. Timers are useful for drivers/pedestrians to plan if there is enough time to attempt to cross the intersection before the light turns red and conversely, the amount of time before the light turns green.

Types

Single aspects

The simplest traffic light comprises either a single or a pair of colored aspects that warns any user of the shared right of way of a possible conflict or danger.
  • Flashing Red: Treat as a stop sign
    Stop sign
    A Stop sign is a traffic sign to notify drivers that they must stop before proceeding.-Specifications:The Vienna Convention on Road Signs and Signals proposed standard stop sign diameters of 0.6, 0.9 or 1.2 metres. UK and New Zealand stop signs are 750, 900 or 1200 mm, according to sign...

    . Also can signal the road is closed.
  • Flashing Amber: Caution, crossing or road hazard ahead
  • Flashing Green: Varies among jurisdiction; can give permission to go straight as well as make a left turn in front of opposing traffic (which is held by a steady red light), or can indicate the end of a green cycle before the light changes to a solid amber.

Dual aspects

These are often seen at railway crossings and at intersections of streets and emergency department driveways. They will flash amber when cross traffic is not expected, and turn red to stop traffic when cross traffic occurs.

Three or more aspects

The universal standard is for the red to be above the green, with amber placed in between. When mounted vertically red is on top, but when mounted horizontally the red light's location depends on the rule of the road, being on the left in a country that drives on the right (e.g. Turkey) and on the right in a country that drives on the left (e.g. Suriname). In addition to the standard red, amber, and green aspects, other aspects are usually mounted when intersection require more control (i.e. public transport, and permissive turns). Traffic lights in this configuration tend to have a delay between any entry points to allow any residual traffic in the intersection to clear. The delay can be dependent upon the type of intersection, traffic, and road conditions.

The three-aspect standard is also used at locks on the Upper Mississippi River in the United States. Red indicates that another vessel which has the right-of-way is currently locking through or will do so soon. Amber indicates that the lock chamber is being filled or emptied to match the pool level of the approaching vessel. Once the lock gates have opened, a green signal indicates that the approaching vessel may enter the lock chamber.

In Quebec, signal bulbs are often mounted horizontally with each color being a distinct shape: red is a square (larger than the conventional circle); yellow is a diamond shape, and green is the usual circle.

Sounds

In some jurisdictions such as Australia, pedestrian lights are associated with a sound device, for the benefit of blind pedestrians. These make a slow beeping sound when the pedestrian lights are red (beep ..... beep ..... beep) and a continuous buzzing sound when the lights are green. In New South Wales
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...

 the sound is produced in the same unit as the push buttons.

European Standard

The European approach to a signalized crossing is use dual or more rarely, a triple aspect. with a blackened out lens of a Pictogram
Pictogram
A pictograph, also called pictogram or pictogramme is an ideogram that conveys its meaning through its pictorial resemblance to a physical object. Pictographs are often used in writing and graphic systems in which the characters are to considerable extent pictorial in appearance.Pictography is a...

 pedestrian. For cyclist, the same approach is used with the lens blackened out for a bicycle frame. It is not uncommon to see lenses with both symbols on them.

Light Sequence;

Green: Cross.

Yellow: Continue to cross only if unable to stop safely.

Flashing Yellow: Cross with caution, usually used when lights are out of order or shut down for the night, for low traffic

Red: Do not cross

Ampelmännchen
Ampelmännchen
is the symbolic person shown on traffic lights at pedestrian crossings in the former German Democratic Republic . Prior to the German reunification in 1990, the two German states had different forms for the Ampelmännchen, with a generic human figure in West Germany, and a generally male figure...

 pedestrian traffic signals have come to be seen as a nostalgic sign for the former German Democratic Republic
German Democratic Republic
The German Democratic Republic , informally called East Germany by West Germany and other countries, was a socialist state established in 1949 in the Soviet zone of occupied Germany, including East Berlin of the Allied-occupied capital city...

.

North American Standard

In North America, the most common aspects found are text only lenses (WALK/ DON'T WALK), and the pictogram stop hand (in Red/Orange) and a walking person (in Green or White). Increasing for retrofits of dual aspects and newer installations, the lower aspect formally used for the walk signal (walking person) is being replaced with a timer countdown.

Light Sequence;

Green or White (Walking Human or WALK); Cross with caution (pedestrians have the right of way; motorists turning left or right must yield to pedestrians when Walking Human or WALK displays)

Flashing Red/Orange (Stop hand or DON'T WALK); Continue to cross if already in the intersection. Do not start to cross

Red/Orange; Do not cross

The Commonwealth of Massachusetts allows an unusual indication variation for pedestrian movement. At signalized intersections without separate pedestrian signal heads the traffic signals may be programmed to turn red in all directions, followed by a steady display of yellow lights simultaneously with the red indications. During this red-plus-yellow indication the intersection is closed to vehicular traffic and pedestrians may cross, usually in whatever direction they choose. Please see Wikipedia- List of Variations in Traffic Light Signalling and Operation for a more complete discussion on unusual traffic signal indications.

Asia

Japan

India

Korea

Green: Cross

Amber: Cross if already in intersection, otherwise do not (however by law, you are allowed to cross)

Red: Do not cross

Red Flashing Traffic Light

Green (sometimes blue) Arrow: Turn only the direction the arrow points
Pedestrians:

Green man: Walk Human

Flashing Green Man: Cross if already in intersection, otherwise do not

Flashing Green Traffic Light

Red Man: Do not cross

Japan generally does not have timers that accompany the signal, however some new signals will have this function.

Some traffic lights in Korea have countdown timers for predestrian crossing lights, usually they countdown from 30 when the green flashing man appears.

Oceania

Some traffic-light controlled junctions have a light sequence that stops all vehicular traffic
Traffic
Traffic on roads may consist of pedestrians, ridden or herded animals, vehicles, streetcars and other conveyances, either singly or together, while using the public way for purposes of travel...

 at the junction at the same time, and gives pedestrians exclusive access to the intersection so that they can cross in any direction (including diagonally). This is known as a pedestrian scramble
Pedestrian scramble
A pedestrian scramble, also known as a 'X' Crossing , diagonal crossing , scramble intersection , and more poetically Barnes Dance, is a pedestrian crossing system that stops all vehicular traffic and allows pedestrians to cross an intersection in every direction, including diagonally, at the same...

 in some places.

Australia:
In Australia, a red man means do not cross, a green man means cross and a red flashing man means finish crossing. Some traffic lights in Melbourne
Melbourne
Melbourne is the capital and most populous city in the state of Victoria, and the second most populous city in Australia. The Melbourne City Centre is the hub of the greater metropolitan area and the Census statistical division—of which "Melbourne" is the common name. As of June 2009, the greater...

 have countdown timers for predestrian crossing lights, usually they countdown from 30 when the red flashing man appears. This however was a trial and was removed as it resulted in large amounts of jaywalking.

Lights for public transport

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

In Portland, Oregon
Portland, Oregon
Portland is a city located in the Pacific Northwest, near the confluence of the Willamette and Columbia rivers in the U.S. state of Oregon. As of the 2010 Census, it had a population of 583,776, making it the 29th most populous city in the United States...

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

In Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

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

In North European countries the tram signals feature white lights of different forms: "S" for "stop", "—" for "caution" and arrows to permit passage in a given direction.
The Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

 use a distinctive "negenoog" (nine-eyed) design shown on the top row of the diagram; bottom row signals are used in Belgium
Belgium
Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...

, France and Germany. The signals mean (from left to right): "go straight ahead", "go left", "go right", "go in any direction" (like the "green" of a normal traffic light), "stop, unless the emergency brake is needed" (equal to "yellow"), and "stop" (equal to "red").

The Hiawatha Line
Hiawatha Line
The Hiawatha Line is a light rail corridor in Hennepin County, Minnesota that extends from downtown Minneapolis to the southern suburb of Bloomington. It was formerly known as the Hiawatha Line named after Hiawatha Avenue. Major connections on the line include the Minneapolis-St...

 in Minneapolis, Minnesota
Minneapolis, Minnesota
Minneapolis , nicknamed "City of Lakes" and the "Mill City," is the county seat of Hennepin County, the largest city in the U.S. state of Minnesota, and the 48th largest in the United States...

 uses a simplified variant of the Belgian/French system in the city's central business district where only the "go" and "stop" configurations are used. A third signal equal to yellow is accomplished by flashing the "go" signal.

In Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

, tram signals are under the regular vehicle signal; however the color of the signal for the tram is coloured orange.

Preemption

Some regions have signals that are interruptible, giving priority to special traffic. Such traffic light preemption is usually reserved for emergency vehicle
Emergency vehicle
An emergency vehicle is any vehicle that is designated and authorized to respond to an emergency. These vehicles are usually operated by designated agencies, often part of the government, but also run by charities, non-governmental organizations and some commercial companies...

s such as fire apparatus
Fire apparatus
A fire apparatus, fire engine, fire truck, or fire appliance is a vehicle designed to assist in fighting fires by transporting firefighters to the scene and providing them with access to the fire, along with water or other equipment...

, ambulance
Ambulance
An ambulance is a vehicle for transportation of sick or injured people to, from or between places of treatment for an illness or injury, and in some instances will also provide out of hospital medical care to the patient...

s, and police
Police
The police is a personification of the state designated to put in practice the enforced law, protect property and reduce civil disorder in civilian matters. Their powers include the legitimized use of force...

 squad car
Squad car
Squad car may refer to:* Police car – The official vehicle of a police authority* Squad Car – An instrumental surf classic by Eddie & the Showmen...

s, though sometimes mass transit vehicles including buses and light rail
Light rail
Light rail or light rail transit is a form of urban rail public transportation that generally has a lower capacity and lower speed than heavy rail and metro systems, but higher capacity and higher speed than traditional street-running tram systems...

 trains can interrupt lights. Most of the systems operate with small transmitters that send radio waves, infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 signals, or strobe light
Strobe light
A strobe light or stroboscopic lamp, commonly called a strobe, is a device used to produce regular flashes of light. It is one of a number of devices that can be used as a stroboscope...

 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.

Turning signals and rules

In some instances, traffic may turn left (in left-driving jurisdictions) or right (in right-driving jurisdictions) after stopping at a red light, providing they give way to the pedestrians and other vehicles. In some places that generally disallow this, a sign next to the traffic light indicates that it is allowed at a particular intersection. Conversely, jurisdictions that generally allow this might forbid it at a particular intersection with a "no turn on red" sign, or put a green arrow to indicate specifically when a turn is allowed without having to yield
Yield sign
In road transport, a ' or ' traffic sign indicates that each driver must prepare to stop if necessary to let a driver on another approach proceed. A driver who stops has yielded the right of way to another...

 to pedestrians (this is usually when traffic from the perpendicular street is making a turn onto one's street and thus no pedestrians are allowed in the intersection anyway). Some jurisdictions allow turning on red in the opposite direction (left in right-driving countries; right in left-driving countries) from a one-way road onto another one-way road; some of these even allow these turns from a two-way road onto a one-way road. Also differing is whether a red arrow prohibits turns; some jurisdictions require a "no turn on red" sign in these cases. A study in the State of Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

 (a right-driving jurisdiction) concluded that allowing drivers to proceed straight on red after stopping, at specially posted T-intersections where the intersecting road went left only, was dangerous. Proceeding straight on red at T-intersections where the intersecting road went left only used to be legal in Mainland China
Mainland China
Mainland China, the Chinese mainland or simply the mainland, is a geopolitical term that refers to the area under the jurisdiction of the People's Republic of China . According to the Taipei-based Mainland Affairs Council, the term excludes the PRC Special Administrative Regions of Hong Kong and...

, 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
Road Traffic Safety Law of the People's Republic of China
The Road Traffic Safety Law of the People's Republic of China is a law which was passed by the National People's Congress of the People's Republic of China on October 28, 2003, promulgated by Decree No...

 took effect on 1 May 2004, such movement was outlawed. In some other countries, the permission is indicated by a flashing amber arrow (cars do not have to stop but must give way to other cars and pedestrians).

Another distinction is between intersections that have dedicated signals for turning across the flow of opposing traffic and those that do not. Such signals are called dedicated left-turn lights in the United States and Canada (since opposing traffic is on the left). With dedicated left turn signals, a left-pointing arrow turns green when traffic may turn left without opposing traffic and pedestrian conflict, and turns red or disappears otherwise. Such a signal is referred to as a "protected" signal if it has its own red phase; a "permissive" signal does not have such a feature. Three standard versions of the permissive signal exist: One version is a horizontal bar with five lights - the green and yellow arrows are located between the standard green and yellow lights. A vertical 5-light bar holds the arrows underneath the standard green light (in this arrangement, the yellow arrow is sometimes omitted, leaving only the green arrow below the solid green light, or possibly an LED
LEd
LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....

 based device capable of showing both green and yellow arrows within a single lamp housing). Some newer LED turn arrows seen in parts of Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 are capable of multicolored animation. Such lights will often display a flashing and animated green or yellow arrow when the dedicated turn is allowed, but then transform into a red arrow on a white background with a red line through it, emphasising that the turn is no longer allowed. These lights will also often have the words "NO TURN" displayed, or an explanatory reason why the turn is not allowed, such as "TRAIN" in the case of a rail or light rail crossing. A third type is known as a "doghouse" or "cluster head" - a vertical column with the two normal lights is on the right side of the signal, a vertical column with the two arrows is located on the left, and the normal red signal is in the middle above the two columns. Cluster signals in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 and New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

 use six signals, the sixth being a red arrow that can operate separately from the standard red light. In a fourth type, sometimes seen at intersections in Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....

 and Quebec, Canada
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....

, there is no dedicated left-turn lamp per se. Instead, the normal green lamp flashes rapidly, indicating permission to go straight as well as make a left turn in front of opposing traffic, which is being held by a steady red lamp. (This "advance green," or flashing green can be somewhat startling and confusing to drivers not familiar with this system. This also can cause confusion amongst visitors to British Columbia, where a flashing green signal denotes a pedestrian controlled intersection.) Another interesting practice seen at least in Ontario is that cars wishing to turn left that arrived after the left turn signal ended can do so during the amber phase, as long as there is enough time to make a safe turn.

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

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

Some intersections with permissive turn signals occasionally have what is known as "yellow trap
Yellow trap
This article assumes traffic drives on the right.The yellow trap is a potentially dangerous scenario in traffic flow through a traffic light relating to permissive left turns. It occurs when a circular yellow light is displayed to a movement with permissive left turns, while at the same time,...

", "lag-trap", or "left turn trap" (in right-driving countries). This refers to situations when left-turning drivers are trapped in the intersection with a red light, while opposing traffic still has a green. In British Columbia, the law addresses this problem by giving a left-turning driver already in an intersection the right-of-way to make the turn once the intersection is otherwise clear, regardless of the traffic light state.

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

Although motorcycle
Motorcycle
A motorcycle is a single-track, two-wheeled motor vehicle. Motorcycles vary considerably depending on the task for which they are designed, such as long distance travel, navigating congested urban traffic, cruising, sport and racing, or off-road conditions.Motorcycles are one of the most...

s and scooters
Scooter (motorcycle)
A scooter is a motorcycle with step-through frame and a platform for the operator's feet. Elements of scooter design have been present in some of the earliest motorcycles, and motorcycles identifiable as scooters have been made from 1914 or earlier...

 in most jurisdictions follow the same traffic signal rules for left turns as do cars and trucks, some places, such as Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...

, 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
Hook turn
A hook turn is a traffic-control mechanism where cars that would normally have to turn across oncoming traffic are made to turn across all lanes of traffic instead....

") is also used at many intersections in the CBD of Melbourne, Australia, where either or both streets carry tramways. This is done so right-turning vehicles (Australia drives on the left) do not block the passage of trams. The system is being extended to the suburbs.

Lane control

Lane-control lights are a specific type of traffic light used to manage traffic on a multi-way road, highway or tollway. Typically, these lights allow or forbid traffic to use one or more of the available lanes by the use of green lights or arrows (to permit) or by red lights or crosses (to prohibit). In the US, lane-control lights are often used to control and/or direct the flow of traffic through toll plazas and highway tunnels, such as during unusually-heavy traffic flow when more lanes may be required in one direction than in the other direction, or during a hurricane evacuation, when the lane signals for all lanes will show green for one direction to assist in more rapid traffic flow from the evacuation site. Lane-control lights are also used at highway weigh stations to direct tractor-trailers and other heavy or oversized vehicles into the proper lanes for weighing, inspection or exit.

In the US, most notably the Southeastern, there often is a "continuous-flow" lane. This lane is protected by a single, constant-green arrow pointing down at the lane(s) (sometimes pointing up) permitting the continuous flow of traffic, without regard to the condition of signals for other lanes or cross streets. Continuous lanes are restricted in that vehicles turning from a side street may not cross over the double-white line to enter the continuous lane, and no lane changes are permitted to the continuous lane from an adjacent lane or from the continuous lane to an adjacent lane, until the double-white line has been passed. Some continuous lanes are protected by a raised curb located between the continuous lane and a normal traffic lane, with white and/or yellow reflective paint or tape, prohibiting turning or adjacent traffic from entering the lane. Continuous-flow traffic lanes are found only at "T" intersections where there is no side street or driveway entrance on the right side of the main thoroughfare; additionally, no pedestrians are permitted to cross the main thoroughfare at intersections with a continuous-flow lane, although crossing at the side street may be permitted. Intersections with continuous-flow lanes will be posted with a white regulatory sign approximately 500 feet before the intersection with the phrase, "Right Lane Continuous Traffic," or other similar wording. If the arrow is extinguished for any reason, whether by malfunction or design, traffic through the continuous lane will revert to the normal traffic pattern for adjacent lanes, except that turning or moving into or out of the restricted lane is still prohibited.

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
Police
The police is a personification of the state designated to put in practice the enforced law, protect property and reduce civil disorder in civilian matters. Their powers include the legitimized use of force...

 to direct traffic. Traffic lights save over 11,000 lives per year.

Some jurisdictions, however, have additional right-of-way signs mounted above, below or next to the traffic lights; these take effect when the lights are no longer active. (In Italy
Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

 as well as some jurisdictions in the US, traffic lights inactive at nighttime emit an amber-colored flashing signal in directions owing priority while the intersecting street emit a flashing red light, requiring drivers to stop before proceeding. In Germany the priority directions will not be illuminated while the intersecting streets will be shown a flashing yellow signal.) In the UK and parts of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, 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
All-way stop
An all-way stop is an intersection system used predominantly in the United States of America, Canada and South Africa where traffic approaching it from all directions is required to stop before proceeding through the intersection...

 intersections.

Dummy lights

In an era when intersections were often controlled by a single traffic signal head, many signals were installed on pedestals in the centers of intersections. Often referred to as "dummy lights," these installations often replaced beacons or "mushrooms" that denoted the centers of intersections and separated opposing traffic, with the infrastructure used for the beacons and mushrooms serving the new "stop and go" type signals.

There are a handful of operational dummy lights still in service. Three are located in New York State
New York
New York is a state in the Northeastern region of the United States. It is the nation's third most populous state. New York is bordered by New Jersey and Pennsylvania to the south, and by Connecticut, Massachusetts and Vermont to the east...

: Beacon
Beacon, New York
Beacon is a city located in Dutchess County, New York, United States. The 2010 census placed the city total population at 15,541. Beacon is part of the Poughkeepsie–Newburgh–Middletown, NY Metropolitan Statistical Area as well as the larger New York–Newark–Bridgeport,...

, Canajoharie
Canajoharie (village), New York
Canajoharie is a village in the town of Canajoharie in Montgomery County, New York, USA. As of the 2000 census, the village had a population of 2,257...

 and Croton-on-Hudson
Croton-on-Hudson, New York
Croton-on-Hudson is a village in Westchester County, New York, United States. The population was 8,070 at the 2010 census. It is located in the town of Cortlandt, in New York City's northern suburbs...

.

After a dummy light was knocked down by a truck in 2010, the City of Coleman Texas decided to preserve and refurbish its last two pedestal mounted dummy lights as part of its historic district preservation efforts.

In 2011 The Arkansas Historic Preservation Program nominated the last remaining pedestal mounted signal in Arkansas, located in Smackover, AR, to be listed in the National Register of Historic Places.

Increases in traffic flows have prompted calls for these types of traffic lights to be removed due to safety concerns, but their historic value has kept these landmarks at their original locations. To serve historic district applications, Teeco Safety Systems of Shreveport, LA, still manufactures replacement fixed 4-way traffic signals for pedestal and overhead span wire installations.

Optics and lighting

Traditionally, incandescent and halogen bulbs were used. Because of the low efficiency of light output and a single point of failure (filament burnout) municipalities are increasingly retrofitting traffic signals with LED arrays
that consume less power, have increased light output, last significantly longer, and in the event of an individual LED failure, still operate albeit with a reduced light output. With the use of optics, the light pattern of an LED array can be comparable
with the pattern of an incandescent or halogen bulb.

Due to the low energy usage aspects of LED lights, these lights pose as a driving risk in some areas during Winter. Incandescent and halogen bulbs are generally warm enough to melt away snow that cover individual lights but as LED lights use a fraction of the energy, they are not warm enough to melt snow that may overlay the lights during winter.

Programmable visibility signals

Signals such as the 3M High Visibility Signal
3M High Visibility Signals
3M High Visibility Signals were a line of traffic signal products produced by the 3M Company from 1969 to 2007. The flagship of the product line was the 3M Model 131 High Visibility Signal.-History:...

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

Using a special aluminum foil-based adhesive tape, these signals are "masked" or programmed via the programming lens so that only certain lanes of traffic will view the indication. At the front of these programmable visibility signals is a 12" fresnel lens, each lens tinted to meet United States Institute of Transportation Engineers
Institute of Transportation Engineers
The Institute of Transportation Engineers or ITE is an international educational and scientific association of transportation professionals who are responsible for meeting mobility and safety needs. ITE was founded in 1930 as the Institute of Traffic Engineers and its first president was Ernest P...

 (ITE) chromaticity and luminance standards. The fresnel lens (3M utilized a square lens masked to create a ball indication, McCain used a circular lens itself) focuses or concentrates the light output created by the lamp, and creates a uniform display of light for the lane in which it is intended. These signals were first developed by the 3M
3M
3M Company , formerly known as the Minnesota Mining and Manufacturing Company, is an American multinational conglomerate corporation based in Maplewood, Minnesota, United States....

 Company in the late 1960s, and were wildly popular in the late 1970s as traffic density increased.

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
Fresnel lens
A Fresnel lens is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design...

 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
3M High Visibility Signals
3M High Visibility Signals were a line of traffic signal products produced by the 3M Company from 1969 to 2007. The flagship of the product line was the 3M Model 131 High Visibility Signal.-History:...

 (dubbed the "Programmed Visibility" signal). Although 3M has recently discontinued the M-131, it became a popular traffic signal for skewed or complex intersections. Today, McCain Traffic Systems (a traffic signal manufacturing company based in Vista, California
Vista, California
Vista is a city in north San Diego County, California. It was incorporated January 28, 1963 and became a charter city on June 13, 2007. Located just seven miles inland from the Pacific Ocean in northern San Diego County, the City of Vista has a Mediterranean climate...

) is the only U.S.-based manufacturer producing true "programmable visibility" traffic signals. In addition to aiming, Fresnel lenses, and louvers, visors and back panels are also useful in areas where sunlight would diminish the contrast and visibility of a signal face.

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

Conventional lighting systems

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

Light design

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

In the United Kingdom, 12 inch lights were implemented only with Mellor Design Signal heads designed by David Mellor. These were designed for symbolic optics to compensate for the light loss caused by the symbol. With the invention of anti-phantom, highly visible SIRA lenses, lights of 8 inches (203.2 mm) could be designed to give the same output as plain lenses, so a larger surface area was unnecessary. Consequently lights of 12 inch are no longer approved for use in the UK and all lights installed on new installations have to be 200 millimetres (8 in) in accordance with TSRGD (Traffic Signs Regulations and General Directions). Exemptions are made for 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.

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
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 (except for the snow country
Snow country (Japan)
Snow country refers to areas in Japan characterized by heavy, long-lasting snowfalls....

 to avoid snow buildups), Malaysia, South Korea
South Korea
The Republic of Korea , , is a sovereign state in East Asia, located on the southern portion of the Korean Peninsula. It is neighbored by the People's Republic of China to the west, Japan to the east, North Korea to the north, and the East China Sea and Republic of China to the south...

, Singapore
Singapore
Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...

, Taiwan
Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...

, Thailand
Thailand
Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...

 and Croatia
Croatia
Croatia , officially the Republic of Croatia , is a unitary democratic parliamentary republic in Europe at the crossroads of the Mitteleuropa, the Balkans, and the Mediterranean. Its capital and largest city is Zagreb. The country is divided into 20 counties and the city of Zagreb. Croatia covers ...

, along with some jurisdictions in the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 (including New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

, Wisconsin
Wisconsin
Wisconsin is a U.S. state located in the north-central United States and is part of the Midwest. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michigan to the northeast, and Lake Superior to the north. Wisconsin's capital is...

, most of Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

, Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

, Nebraska
Nebraska
Nebraska is a state on the Great Plains of the Midwestern United States. The state's capital is Lincoln and its largest city is Omaha, on the Missouri River....

, and some localities in Kansas
Kansas
Kansas is a US state located in the Midwestern United States. It is named after the Kansas River which flows through it, which in turn was named after the Kansa Native American tribe, which inhabited the area. The tribe's name is often said to mean "people of the wind" or "people of the south...

, New Jersey
New Jersey
New Jersey is a state in the Northeastern and Middle Atlantic regions of the United States. , its population was 8,791,894. It is bordered on the north and east by the state of New York, on the southeast and south by the Atlantic Ocean, on the west by Pennsylvania and on the southwest by Delaware...

 and Louisiana
Louisiana
Louisiana is a state located in the southern region of the United States of America. Its capital is Baton Rouge and largest city is New Orleans. Louisiana is the only state in the U.S. with political subdivisions termed parishes, which are local governments equivalent to counties...

, most notably New Orleans.), the provinces of Alberta
Alberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...

, Saskatchewan
Saskatchewan
Saskatchewan is a prairie province in Canada, which has an area of . Saskatchewan is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, and on the south by the U.S. states of Montana and North Dakota....

, Québec
Quebec
Quebec or is a province in east-central Canada. It is the only Canadian province with a predominantly French-speaking population and the only one whose sole official language is French at the provincial level....

, Nova Scotia
Nova Scotia
Nova Scotia is one of Canada's three Maritime provinces and is the most populous province in Atlantic Canada. The name of the province is Latin for "New Scotland," but "Nova Scotia" is the recognized, English-language name of the province. The provincial capital is Halifax. Nova Scotia is the...

, some of New Brunswick
New Brunswick
New Brunswick is one of Canada's three Maritime provinces and is the only province in the federation that is constitutionally bilingual . The provincial capital is Fredericton and Saint John is the most populous city. Greater Moncton is the largest Census Metropolitan Area...

 and Newfoundland and Labrador
Newfoundland and Labrador
Newfoundland and Labrador is the easternmost province of Canada. Situated in the country's Atlantic region, it incorporates the island of Newfoundland and mainland Labrador with a combined area of . As of April 2011, the province's estimated population is 508,400...

 and the territories of Northwest Territories
Northwest Territories
The Northwest Territories is a federal territory of Canada.Located in northern Canada, the territory borders Canada's two other territories, Yukon to the west and Nunavut to the east, and three provinces: British Columbia to the southwest, and Alberta and Saskatchewan to the south...

 and Yukon
Yukon
Yukon is the westernmost and smallest of Canada's three federal territories. It was named after the Yukon River. The word Yukon means "Great River" in Gwich’in....

 in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

, as well as some places in Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

 mount lights with their multiple faces arranged horizontally, often with supplemental vertical signals on the side, while others use vertical signals almost exclusively. Horizontal signals have consistent orientation, like their vertical counterparts. Often, supplemental curb pedestal mounts, intended to support a signal for a different approach road, are used when primary signals are partially obscured due to structures such as overpasses, approaches around a building that obscures the primary signal mountings, and unusual approach geometry. In Florida, horizontal signals mounted on poles, known as "mast arms", are in wide use due to their lower wind profile, important for minimising hurricane damage. The horizontally mounted traffic light position is very common in Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

 (with the exceptions of Amarillo, Fort Worth, San Antonio (If one is not counting downtown, access roads, or Universal City) and most of downtown Dallas), New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

, Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

 (except for Orlando
Orlando
Orlando is a major city in the U.S. state of Florida.Orlando may also refer to-Places:* in Florida** Orlando, a major city** Greater Orlando, the 27th-largest metropolitan area in the United States...

, Clearwater
Clearwater, Florida
Clearwater is a city located in Pinellas County, Florida, US, nearly due west of Tampa and northwest of St. Petersburg. In the west of Clearwater lies the Gulf of Mexico and in the east lies Tampa Bay. As of the 2000 census, the city had a total population of 108,787. It is the county seat of...

, St. Petersburg
St. Petersburg, Florida
St. Petersburg is a city in Pinellas County, Florida, United States. It is known as a vacation destination for both American and foreign tourists. As of 2008, the population estimate by the U.S. Census Bureau is 245,314, making St...

, Daytona Beach, Fort Lauderdale and intersections with wire-span assemblies primarily used in Gainesville
Gainesville, Florida
Gainesville is the largest city in, and the county seat of, Alachua County, Florida, United States as well as the principal city of the Gainesville, Florida Metropolitan Statistical Area . The preliminary 2010 Census population count for Gainesville is 124,354. Gainesville is home to the sixth...

, Ocala, Tampa
Tâmpa
Tâmpa may refer to several villages in Romania:* Tâmpa, a village in Băcia Commune, Hunedoara County* Tâmpa, a village in Miercurea Nirajului, Mureş County* Tâmpa, a mountain in Braşov city...

 and West Palm Beach), and Nebraska
Nebraska
Nebraska is a state on the Great Plains of the Midwestern United States. The state's capital is Lincoln and its largest city is Omaha, on the Missouri River....

 (except for Omaha
Omaha
Omaha may refer to:*Omaha , a Native American tribe that currently resides in the northeastern part of the U.S. state of Nebraska-Places:United States* Omaha, Nebraska* Omaha, Arkansas* Omaha, Georgia* Omaha, Illinois* Omaha, Texas...

 and intersections with wire-span assemblies) on most intersections. In Wisconsin
Wisconsin
Wisconsin is a U.S. state located in the north-central United States and is part of the Midwest. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michigan to the northeast, and Lake Superior to the north. Wisconsin's capital is...

 horizontally mounted traffic lights are used almost exclusively, however, in recent years, some vertically mounted lights have started popping up at some intersections. In areas where wind-load is not as major a concern as ice-load such as Illinois
Illinois
Illinois is the fifth-most populous state of the United States of America, and is often noted for being a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal,...

 or Minnesota
Minnesota
Minnesota is a U.S. state located in the Midwestern United States. The twelfth largest state of the U.S., it is the twenty-first most populous, with 5.3 million residents. Minnesota was carved out of the eastern half of the Minnesota Territory and admitted to the Union as the thirty-second state...

 the lights are mounted vertically to reduce the accumulation of ice or snow over the surface of the signal heads. In downtown Cincinnati, a majority of the lights are mounted horizontally with some vertical ones on the side while the rest of Ohio
Ohio
Ohio is a Midwestern state in the United States. The 34th largest state by area in the U.S.,it is the 7th‑most populous with over 11.5 million residents, containing several major American cities and seven metropolitan areas with populations of 500,000 or more.The state's capital is Columbus...

 uses verticals.

Traffic signals in most areas of Europe are located at the stop line on same side of the intersection as the approaching traffic (there being both right- and left-hand traffic) and are often mounted overhead as well as on side of the road. At particularly busy junctions for freight, higher lights may be mounted specifically for trucks
Lorry
-Transport:* Lorry or truck, a large motor vehicle* Lorry, or a Mine car in USA: an open gondola with a tipping trough* Lorry , a horse-drawn low-loading trolley-In fiction:...

. The stop line alignment is done to prevent vehicles blocking any crosswalk and allow for better pedestrian traffic flow. There may also be a special area a few meters in advance of the stop line where cyclists may legally wait but not motor vehicles; this advanced stop line
Advanced stop line
An advanced stop line , also called advanced stop box or bike box, these are road markings at signalised road junctions allowing certain types of vehicle a head start when the traffic signal changes from red to green.-Description:...

 is often painted with a different road surface with greater friction and a high color, both for the benefit of cyclists and for other vehicles. The traffic lights are mounted so that cyclists can still see them.

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

Implementation

According to transportation engineers, traffic lights can have both positive and negative effects on traffic safety and traffic flow. The separation of conflicting streams of traffic in time can reduce the chances of right-angle collisions. But also the frequency of rear-end crashes can be increased by the installation of traffic lights, and they can adversely affect the safety of bicycle and pedestrian traffic. They can increase the traffic capacity at intersections, but can also result in excessive traffic delay. Hans Monderman
Hans Monderman
Hans Monderman was a Dutch road traffic engineer and innovator. He was recognized forradically challenging criteria used to evaluate engineering solutions for street design...

, the innovative Dutch traffic engineer, and pioneer of shared space
Shared space
Shared space is an urban design concept aimed at integrated use of public spaces. It encourages traffic engineers, urban planners and experts from other fields to consult with users of public space when planning and designing streets and squares in both built and non-built environments...

 schemes, was sceptical of their role, and is quoted as having said of them: "We only want traffic lights where they are useful and I haven't found anywhere where they are useful yet".

Small vehicle safety

In some instances the stoplight traffic detector will not change the light for small vehicles such as motorcycles, scooters and particularly bicycles, because the induction loop
Induction loop
Induction loop is a term used to describe an electromagnetic communication- and detection system, relying on the fact that a moving magnet will induce an electrical current in a nearby conducting wire. Induction loops are used for transmission and reception of communication signals, or for...

 sensor fails to detect the small vehicle. A vehicle with sufficient mass of metal such as steel interacts with the sensors magnetic field causing the light to change at the appropriate time. Motorcycles and scooters have much less mass than cars, resulting in a failure to trigger the sensor, while bicycles may not even be constructed with metal. This situation most often occurs at the times of day when other traffic is sparse as well as when the small vehicle is coming from a direction that does not have a high volume of traffic. Not all sensor traffic lights have this problem. There are some that can pick up these vehicles without problems and most are capable of doing so. However, through road construction and other processes, they are not properly adjusted or calibrated for the small vehicles. Magnetic field sensors detect vehicles passing over them and not necessarily stationary vehicles.

Some states in the United States have passed laws which give motorcyclists the right to proceed past a red light with caution after stopping when the device which causes the light to change from red to green does not activate.

Traffic signal warrants

Traffic signals have strengths and weaknesses that must be considered when deciding whether to install them. Signaled intersections can reduce delay for side road traffic and reduce the occurrence of collisions by turning traffic and cross traffic. But they may also cause delay for traffic on the main road, and often increase rear-end collision
Rear-end collision
A rear-end collision is a traffic accident wherein a vehicle crashes into the vehicle in front of it, usually caused by tailgating or panic stops...

s by up to 50%. Since right-angled and turn-against-traffic collisions are more likely to result in injuries, this is often an acceptable trade-off.

US Department of Transportation Manual of Uniform Traffic Control Devices

Criteria have been developed to help ensure that new signals are installed only where they will do more good than harm. In the United States these criteria are called warrants, and are found in the Manual on Uniform Traffic Control Devices
Manual on Uniform Traffic Control Devices
The Manual on Uniform Traffic Control Devices is a document issued by the Federal Highway Administration of the United States Department of Transportation to specify the standards by which traffic signs, road surface markings, and signals are designed, installed, and used...

 (MUTCD), the Federal regulation covering the use of signs, pavement markings, traffic signals, and similar devices.

In the US MUTCD, there are nine warrants for traffic signals:
  • Eight-Hour Vehicular Volume. Traffic volume must exceed prescribed minima for eight hours of an average day (usually a weekday).

  • Four-Hour Vehicular Volume. Traffic volume must exceed prescribed minima for four hours of an average day.
  • Peak Hour Volume or Delay. This is applied only in unusual cases, such as office parks, industrial complexes, or park and ride
    Park and ride
    Park and ride facilities are car parks with connections to public transport that allow commuters and other people wishing to travel into city centres to leave their vehicles and transfer to a bus, rail system , or carpool for the rest of their trip...

     lots that attract or discharge large numbers of vehicles in a short time, and, for a minimum of one hour of an average day. The side road traffic suffers undue delay when entering or crossing the major street.
  • Pedestrian Volume, if the traffic volume on a major street is so heavy that pedestrians experience excessive delay in attempting to cross it
  • School Crossing, if the traffic density at school crossing times exceeds one per minute which is considered to provide too few gaps in the traffic for children to safely cross the street
  • Coordinated Signal System, for places where adjacent traffic control signals do not keep traffic grouped together efficiently.
  • Crash Experience. The volumes in the eight- and four-hour warrants may be reduced if five or more right-angle and cross traffic turn collisions have happened at the intersection in a twelve month period.
  • Roadway Network. Installing a traffic control signal at some intersections might be justified to encourage concentration and organization of traffic flow on a roadway network.
  • Intersection Near a Grade Crossing. A traffic control signal is often justified at an intersection near a railroad crossing, in order to provide a preemption sequence to allow traffic queued up on the tracks an opportunity to clear the tracks before the train arrives.


An intersection should meet one or more of these warrants before a signal is installed. However, meeting one or more warrants does not require the installation of a traffic signal, only suggests that they may be suitable. It could be that a roundabout
Roundabout
A roundabout is the name for a road junction in which traffic moves in one direction around a central island. The word dates from the early 20th century. Roundabouts are common in many countries around the world...

 would work better there. There may be other unconsidered conditions that lead to traffic engineers to conclude a signal is undesirable. For example, they may decide not to install a signal at an intersection if traffic stopped by it will back up and block another, more heavily trafficked intersection. Also, if a signal meets only the peak hour warrant, the advantages during that time may not outweigh the disadvantages during the rest of the day.

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

Legal implications

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

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

Jurisdictions differ somewhat on how to deal with "red light running" — attempts by motorists to race to an intersection while facing a yellow light, in an attempt to beat the red. In some locales, as long as the light is yellow when the motorist enters the intersection, no offense has been committed; in others, if the light turns red at any time before the motorist clears the intersection, then an offense occurs. In Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...

 and other places, a stricter standard applies-—running a yellow (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
New South Wales
New South Wales is a state of :Australia, located in the east of the country. It is bordered by Queensland, Victoria and South Australia to the north, south and west respectively. To the east, the state is bordered by the Tasman Sea, which forms part of the Pacific Ocean. New South Wales...

 are activated only if a motorist enters an intersection 0.3 seconds after the light has turned red.

In some jurisdictions (such as Toronto
Toronto
Toronto is the provincial capital of Ontario and the largest city in Canada. It is located in Southern Ontario on the northwestern shore of Lake Ontario. A relatively modern city, Toronto's history dates back to the late-18th century, when its land was first purchased by the British monarchy from...

, Washington D.C., New York City
New York City
New York is the most populous city in the United States and the center of the New York Metropolitan Area, one of the most populous metropolitan areas in the world. New York exerts a significant impact upon global commerce, finance, media, art, fashion, research, technology, education, and...

, and the state of California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

), there are ordinances against "gridlock
Gridlock
The term gridlock is defined as "A state of severe road congestion arising when continuous queues of vehicles block an entire network of intersecting streets, bringing traffic in all directions to a complete standstill; a traffic jam of this kind." The term originates from a situation possible in...

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

In Sackville
Sackville, New Brunswick
Sackville is a Canadian town in Westmorland County, New Brunswick.Mount Allison University is located in the town...

, New Brunswick
New Brunswick
New Brunswick is one of Canada's three Maritime provinces and is the only province in the federation that is constitutionally bilingual . The provincial capital is Fredericton and Saint John is the most populous city. Greater Moncton is the largest Census Metropolitan Area...

 it is customary for through traffic to voluntarily yield to the first oncoming left-turning vehicle to allow it to perform a Pittsburgh left
Pittsburgh Left
The Pittsburgh left is a colloquial term for the driving practice of the first left-turning vehicle taking precedence over vehicles going straight through an intersection, associated with the Pittsburgh, Pennsylvania area. The practice is also referred to as a Boston left...

. This is similar to a hook turn
Hook turn
A hook turn is a traffic-control mechanism where cars that would normally have to turn across oncoming traffic are made to turn across all lanes of traffic instead....

 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, 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 camera
Red light camera
A red light camera is a traffic enforcement camera that captures an image of a vehicle which has entered an intersection against a red traffic light. By automatically photographing vehicles that run red lights, the camera produces evidence that assists authorities in their enforcement of traffic laws...

s 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 is photographed so that they can be fined for the violation. In some jurisdictions in the United States, private companies have been contracted to operate traffic-related cameras and in turn receive a portion of the resulting revenues. In some cases red light cameras have been abused by local governments, where vehicle operators have been fined as a result of traffic systems that have been improperly modified.

Light timing length

The length of amber lights can differ, for example in many places the length of an amber light is usually 5 seconds, but elsewhere it may be as little as 3 seconds, considerably reducing the time for reaction. It is typical for these times to vary according to the set speed limit, with longer times for higher limits. In the state of Georgia, United States, an amber light must be lit one second for every 10 miles per hour (4.5 m/s) of posted speed limit. In this situation, a 45 miles per hour (20.1 m/s) posted speed limit would have a 4.5 second amber light. For intersections with red light cameras, 1 extra second must be added. In Colorado Springs, Colorado, green lights at numerous intersections have been noted with durations of approximately two seconds. In the United States, there is a recommended federal safety minimum of three seconds for yellow lights.

The time from when a red light is displayed and when a cross street is given a green light is usually based on the physical size of the intersection. This intervening period is called the all-red time. A typical all-red time is 3 seconds to allow cars to clear the intersection. In a wider intersection, such as 4 lane road or highway intersection, the all-red time may be as much as 5 seconds, allowing drivers who could not or would not stop at the amber light enough time to clear the intersection without causing a collision.

In other contexts

The symbolism of a traffic light (and the meanings of the three primary colors used in traffic lights) are frequently found in many other contexts. Since they are often used as single spots of color without the context of vertical position, they are typically not comprehensible to up to one in ten males who are color blind
Color blindness
Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...

.

Traffic lights have also been used in pieces of artwork, particularly the Traffic Light tree
Traffic Light tree
The Traffic Light tree was created by French sculptor Pierre Vivant following a competition run by the Public Art Commissions Agency. It is situated on a roundabout near Canary Wharf, at the junctions of Heron Quay Bank, Marsh Wall and Westferry Road...

 in London, UK.

Racing

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

Scuderia Ferrari
Scuderia Ferrari
Scuderia Ferrari is the racing team division of the Ferrari automobile marque. The team currently only races in Formula One but has competed in numerous classes of motorsport since its formation in 1929, including sportscar racing....

, a Formula One
Formula One
Formula One, also known as Formula 1 or F1 and referred to officially as the FIA Formula One World Championship, is the highest class of single seater auto racing sanctioned by the Fédération Internationale de l'Automobile . The "formula" designation in the name refers to a set of rules with which...

 racing team, formerly used a traffic light system during their pit stops to signal to their drivers to when to leave the pits. The red light was on when the tires were being changed and fuel was being added, yellow was on when the tires were changed, and green was on when all work was completed. The system is (usually) completely automatic. However, the system was withdrawn after the 2008 Singapore Grand Prix
Singapore Grand Prix
The Singapore Grand Prix is a motor race, currently in the calendar of the FIA Formula One World Championship. It is currently held in the Marina Bay area of Singapore...

, due to the fact that it heavily delayed Felipe Massa
Felipe Massa
Felipe Massa is a Brazilian Formula One racing driver. He finished second in the Drivers' World Championship, and is under contract to race for Scuderia Ferrari until the end of the season.-Early years:...

 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
Adrian Sutil
-Biography:Sutil is the son of professional musicians Monika, a German, and Jorge, a Uruguayan. He has two brothers named Daniel and Raphael. He weighs 165 pounds and is 6 feet tall. A talented pianist, Sutil started karting at 14 before moving up to Swiss Formula Ford 1800 in 2002...

, earning him a penalty. He finally stopped at the end of the pit lane, forcing Ferrari's mechanics to sprint down the whole of the pit lane to remove the hose. As a result of this, and the penalty he also incurred, Massa finished 13th. Ferrari
Ferrari
Ferrari S.p.A. is an Italian sports car manufacturer based in Maranello, Italy. Founded by Enzo Ferrari in 1929, as Scuderia Ferrari, the company sponsored drivers and manufactured race cars before moving into production of street-legal vehicles as Ferrari S.p.A. in 1947...

 decided to use a traditional "lollipop" for the remainder of the 2008 season.
2008 Formula One season
The 2008 Formula One season was the 59th FIA Formula One World Championship season. It began on 16 March and ended on 2 November with eighteen Grand Prix races....


As a rating mechanism

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

See also

  • Ampelmännchen
    Ampelmännchen
    is the symbolic person shown on traffic lights at pedestrian crossings in the former German Democratic Republic . Prior to the German reunification in 1990, the two German states had different forms for the Ampelmännchen, with a generic human figure in West Germany, and a generally male figure...

  • Level crossing
    Level crossing
    A level crossing occurs where a railway line is intersected by a road or path onone level, without recourse to a bridge or tunnel. It is a type of at-grade intersection. The term also applies when a light rail line with separate right-of-way or reserved track crosses a road in the same fashion...

  • Pedestrian crossing
    Pedestrian crossing
    A pedestrian crossing or crosswalk is a designated point on a road at which some means are employed to assist pedestrians wishing to cross. They are designed to keep pedestrians together where they can be seen by motorists, and where they can cross most safely across the flow of vehicular traffic...

  • Traffic optimization
    Traffic optimization
    - Need for traffic optimization :Texas Transportation Institute estimates travel delays of 220,000,000 hours all over the U.S. and between 17–55 hours of delay per person relating to congestion on the streets. Traffic device optimization hence becomes a significant aspect of operations.-...

  • Yellow trap
    Yellow trap
    This article assumes traffic drives on the right.The yellow trap is a potentially dangerous scenario in traffic flow through a traffic light relating to permissive left turns. It occurs when a circular yellow light is displayed to a movement with permissive left turns, while at the same time,...


External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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