Traffic light

The 1968 Vienna Convention on Road Signs and Signals Chapter III provides international standards for the setup of traffic signal operations. Not all states have ratified the convention.

A three-colour signal head should have three non-flashing lights which are red, yellow, and green, either arranged horizontally (on the side opposite to the direction of traffic) or vertically (with red on top). A two-colour signal head may be used in temporary operation and consists of red and green non-flashing lights. In both cases, all lights should be circular or arrow-shaped.[19]

Permissible signals for regulating vehicle traffic (other than public transport vehicles) are outlined in Article 23:[19]

Green arrows are added to signals to indicate that drivers can travel in a particular direction, while the main lights for that approach are red, or that drivers can only travel in one particular direction. Alternatively, when combined with another green signal, they may indicate that turning traffic has priority over oncoming traffic (known as a "filter arrow").[18]^: 3.5

The United States is not party to the Convention and the Manual on Uniform Traffic Control Devices (MUTCD) outlines correct operation in that country. The MUTCD specifies the following vehicular signals:[20]

In the US, a single signal head shall have three, four, or five aspects (though a single aspect green arrow may be displayed to indicate a continuous movement). The signals must be arranged red, yellow, and green vertically (top to bottom) or horizontally (left to right).[20]

In the US, a single-aspect flashing yellow signal can be used to raise attention to a warning sign and a single-aspect flashing red signal can be used to raise attention to a STOP, DO NOT ENTER, or WRONG WAY sign.[20] Flashing red or amber lights, known as intersection control beacons, are used to reinforce stop signs at intersections.[21]

Generally, at least one direction of traffic at an intersection has a green light (green aspect) at any moment in the cycle. In some jurisdictions, for a brief time, all signals at an intersection show red at the same time, to clear any traffic in the intersection. The delay can depend on traffic, road conditions, the physical layout of the intersection, and legal requirements. Thus modern signals are built to allow the "all red" in an intersection, even if the feature is not used.

Some signals have no "all red" phase: the light turns green for cross traffic the instant the other light turns red.[note 1]

Another variant in some locations is the pedestrian scramble, where all the traffic lights for vehicles become red, and pedestrians are allowed to walk freely, even diagonally, across the intersection. Use of the Pedestrian signals (see below) will alter the traffic signal cycle.

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

In the Canadian province of Quebec and the Maritime provinces, lights are often arranged horizontally, but each aspect is a different shape: red is a square (larger than the normal circle) and usually in pairs at either end of the fixture, yellow is a diamond, and green is a circle. In many southern and southwestern U.S. states, most traffic signals are similarly horizontal in order to ease wind resistance during storms and hurricanes.[22]

Japanese traffic signals mostly follow the same rule except that the "go" signals are referred to as 青 (blue), which they historically were in fact, but this caused complications with the international "green for go" rule, so [23] in 1973 a decree was issued that the "go" light should be changed to the bluest possible hue of green, thus making it factually greener without having to change the name from 青(blue) to 緑(green).

In the UK, normal traffic lights follow this sequence:[24]

• Red – Stop, do not proceed
• Red and Yellow - Get ready to proceed, but do not proceed yet
• Green – Proceed if the intersection or crossing is clear, vehicles are not allowed to block the intersection or crossing
• Yellow - Stop, unless it is unsafe to do so.

A speed sign is a special traffic light, variable traffic sign, or variable-message sign giving drivers a recommended speed to approach the next traffic light in its green phase[25] and avoid a stop due to reaching the intersection when lights are red.[26][note 2]

The three-aspect standard is also used at locks on the Upper Mississippi River. Red means that another vessel is passing through. Amber means that the lock chamber is being emptied or filled to match the level of the approaching vessel. After the gate opens, green means that the vessel may enter.

Railroad signals, for stopping trains in their own right of way, generally use the opposite positioning of the colours; that is, for signals above the driver's eyeline, green on top and red below is the standard placement of the signal colours on railroad tracks. There are three reasons for this variation: there is no risk that railway signals will be masked by a tall vehicle between the driver and the signal; train speeds in fog are much higher than for road vehicles, so it is important that the most restrictive signal is closest to the driver's eyeline; and with railway signals often in exposed rural locations, there is a risk of any signal other than the bottom one being masked by snow building up on the hood of the signal below.

Where pedestrians need to cross the road between junctions, a signal-controlled crossing may be provided as an alternative to a zebra crossing or uncontrolled crossing. Traffic lights are normally used at crossings where vehicle speeds are high, where either vehicle or pedestrian flows are high or near signalised junctions.[18]^: c.18

In the UK, this type of crossing is called a pelican crossing, though more modern iterations are puffin and pedex crossings. In the UK, these crossings normally need at least four traffic signals, which are of a regular type (red, amber, and green), two facing in each direction. Furthermore, pedestrians will be provided with push buttons and pedestrian signals, consisting of a red and green man. Farside signals are located across the crossing, while nearside signals are located below the traffic lights, facing in the direction of oncoming traffic.[18]^: c.18

A HAWK beacon is a special type of traffic used in the US at mid-block crossings. These consist of two red signals above a single yellow signal. The beacon is unlit until a pedestrian pushes the cross button. Then a yellow light will show, followed by both red lights, at which point the 'Walk' symbol will illuminate for pedestrians. At the end of the crossing phase, the 'Don't Walk' symbol will flash, as will the yellow traffic light.[28]

Pedestrians are usually incorporated into urban signalised junctions in one of four ways: no facilities, parallel walk, walk with traffic, or all-red phases.

No facilities may be provided if pedestrian demand is low, in areas where pedestrians are not permitted, or if there is a subway or overpass. No provision of formal facilities means pedestrians will have to self-evaluate when it is safe to cross, which can be intimidating for pedestrians.[18]^: 71

With a 'parallel walk' design, pedestrians walk alongside the traffic flow. A leading pedestrian interval may be provided, whereby pedestrians get a 'walk' signal before the traffic gets a green light, allowing pedestrians to establish themselves on the crossing before vehicles begin to turn, to encourage drivers to give way.[29]

A 'walk with traffic' facility allows pedestrians to go at the same time as other traffic movements with no conflict between movements. This can work well on one-way roads, where turning movements are banned or where the straight-ahead movement runs in a different stage from the turning movement. A splitter island could also be provided. Traffic will pass on either side of the island and pedestrians can cross the road safely between the other flows.[18]^: 72

The pedestrian scramble at Shibuya, Tokyo

An all-red stage, also known as a full pedestrian stage or a pedestrian scramble, holds all vehicular traffic at the junction to allow pedestrians time to safely cross. This may require a longer cycle time and increase pedestrian wait periods. This can be eased by providing two pedestrian stages.[18]^: 71–72 This also allows diagonal crossings to be provided, known as a pedestrian scramble or the Barnes Dance, such as the famous Shibuya crossing in Tokyo. The Barnes Dance is named after an American traffic engineer, Henry A. Barnes. Barnes did not claim to have invented the system but was a strong advocate of it, having observed the difficulties his daughter faced crossing the road to get to school.[30]

A diagram of a countdown timer in the US style

Pedestrian countdown timers are becoming common at urban signal-controlled crossings. Where a pedestrian countdown is shown, it is normally used in conjunction with the flashing hand signal (in the US and Canada) or blackout period (UK), showing the amount of time remaining in seconds until the end of the flashing hand or blackout.[18][31]

Pedestrian countdown timers do not significantly increase or reduce the number of red- and amber-light runners (drivers). Studies have found that pedestrian countdown timers do significantly improve pedestrian compliance over traditional pedestrian signals; however, results are mixed.[31][32]

Traffic light animation (pedestrians, cyclists and traffic) in Ljubljana, Slovenia

In some jurisdictions such as Australia, pedestrian lights are associated with a sound device, for the benefit of blind and visually impaired pedestrians. These make a slow beeping sound when the pedestrian lights are red and a continuous buzzing or fast beeping sound when the lights are green. In the Australian States of Queensland, New South Wales, Victoria, and Western Australia, the sound is produced in the same unit as the push buttons. In a circle above the button, the sound is produced and can be felt along with a raised arrow that points in the direction to walk.[36] This system of assistive technology is also widely used at busy intersections in Canadian cities.

In the United Kingdom, the Puffin crossings and their predecessor, the Pelican crossing, will make a fast beeping sound to indicate that it is safe to cross the road. The beeping sound is disabled during the nighttime so as not to disturb any nearby residents.[37]

In some states in the United States, at some busy intersections, buttons will make a beeping sound for blind people. When the light changes, a speaker built into the button will play a recording to notify blind people that it is safe to cross. When the signal flashes red, the recording will start to count down with the countdown timer.

In several countries such as New Zealand, technology also allows deaf and blind people to feel when lights have changed to allow safe crossing. A small pad, housed within an indentation in the base of the box housing the button mechanism, moves downwards when the lights change to allow crossing. This is designed to be felt by anyone waiting to cross who has limited ability to detect sight or sound.

In Japan, a traffic light emits an electronic sound that mimics the sound of birdsong to help the visually impaired. Some traffic lights fix the order and type of sound so that they can tell which direction is a green light. In general, "Piyo" (peep) and "Piyo-piyo", which is a small bird call, and "Kakkō" and "Ka-kakkō", which is a cuckoo call, are associated with this system.[38]

Some pedestrian crossings in Lithuania make a slowish beeping sound indicating that the traffic light is about to turn off.

Some systems use the letter B for buses and T for trams. The METRO light rail system in Minneapolis, Minnesota, the Valley Metro Rail in Phoenix, Arizona, and the RTA Streetcar System in New Orleans use a simplified variant of the Belgian/French system in the respective city's central business district where only the "go" and "stop" configurations are used. A third signal equal to amber is accomplished by flashing the "go" signal.

Traffic lights for public transport in the Netherlands and Belgium

In some European countries and Russia, dedicated traffic signals for public transport (tram, as well any that is using a dedicated lane) 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 the case of a tram signal, if there are no tram junctions or turns at an intersection, a simpler system of one amber signal in the form of the 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.[43]

The Netherlands uses a distinctive "negenoog" (nine-eyed) design shown on the top row of the diagram;[44] bottom row signals are used in Belgium, Luxembourg, 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").

In Japan, tram signals are under the regular vehicle signal; however, the colour of the signal intended for trams is orange. In Sweden, All signals use white lighting and special symbols ("S", "–" and an arrow) to distinguish them from regular signals. The small light at the top tells the driver when the vehicle's transponder signal is received by the traffic light. In Hong Kong, an amber T-signal is used for trams, in place of the green signal. In addition, at any tramway junction, another set of signals is available to indicate the direction of the tracks. In Australia and New Zealand, a white "B" or "T" sometimes replaces the green light indicating that buses or trams (respectively) have right of way.

• 
  T signal (trams) in Hong Kong
• 
  Tram signals, Geneva
• 
  Bus and tram signals in Karlsruhe, Germany
• 
  Swedish tram signal
• 
  Toronto 'Spadina' signals (black signals for transport)
• 
  Tram signal showing 'stop' in Moscow, Russia
• 
  A bus traffic light in Newport, United Kingdom

Some regions have signals that are interruptible, giving priority to special traffic, usually emergency vehicles such as fire apparatus, ambulances, and police squad cars.[45][46] Most of the systems operate with small transmitters that send radio waves, infrared signals, or strobe light signals that are received by a sensor on or near the traffic lights. Some systems use audio detection, where a certain type of siren must be used and detected by a receiver on the traffic light structure.

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

In lieu of preemptive mechanisms, in most jurisdictions, emergency vehicles are not required to respect traffic lights. However, emergency vehicles must slow down, proceed cautiously and activate their emergency lights to alert oncoming drivers to the preemption when crossing an intersection against the light.[47][48]

Unlike preemption, which immediately interrupts a signal's normal operation to serve the preempting vehicle and is usually reserved for emergency use, "priority" is a set of strategies intended to reduce delay for specific vehicles, especially mass transit vehicles such as buses. A variety of strategies exist to give priority to transit but they all generally work by detecting approaching transit vehicles and making small adjustments to the signal timing. These adjustments are designed to either decrease the likelihood that the transit vehicle will arrive during a red interval or decrease the length of the red interval for those vehicles that are stopped. Priority does not guarantee that transit vehicles always get a green light the instant they arrive as preemption does.

A traffic light in Westbrook, Maine, on State Route 25. Notice the red arrow to the left of the two green straight lights.

Some jurisdictions, particularly in North America, permit drivers to make a kerb-to-kerb turn (i.e. not crossing other traffic streams) at a red light, provided they first come to a complete stop and give way to other traffic and pedestrians. The rule has been criticised for endangering pedestrians, and some highway authorities have banned turning on red at specific junctions.[49]

Arrow aspects may be used to permit certain movements or convey other messages to road users.

A green arrow may display to require drivers to turn in a particular direction only or to allow drivers to continue in a particular direction when the signal is red.[18]^: 3.5 Generally, a green phase is illuminated at the beginning of the green phase (a "leading turn") or at the end of the green phase (a "lagging turn").

An 'indicative arrow' may be displayed alongside a green light. This indicates to drivers that oncoming traffic is stopped, such that they do not need to give way to that traffic when turning across it. As right-turning traffic (left-side drive) or left-turning traffic (right-side drive) does not normally have priority, this arrow is used to allow turning traffic to clear before the next phase begins.[18]^: 3.5

Some variations exist on this set up:

• 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 five-light bar holds the arrows underneath the standard green light (in this arrangement, the amber arrow is sometimes omitted, leaving only the green arrow below the steady green light, or possibly an LED-based device capable of showing both green and amber arrows within a single lamp housing).
• Some newer LED turn arrows seen in parts of Canada are capable of multicoloured animation. Such lights will often display a flashing and animated green or yellow arrow when the dedicated turn is allowed but then transform into a red arrow on a white background with a red line through it, emphasising that the turn is no longer allowed. These lights will also often have the words "no turn" displayed, or an explanatory reason why the turn is not allowed, such as "train" in the case of a rail or light rail crossing.
• A third type is known as a "doghouse" or "cluster head" – a vertical column with the two normal lights is on the right side of the signal, a vertical column with the two arrows is located on the left, and the normal red signal is in the middle above the two columns. Cluster signals in Australia and New Zealand use six signals, the sixth being a red arrow that can operate separately from the standard red light.
• In a fourth type, sometimes seen at intersections in Ontario and Quebec, Canada, there is no dedicated left-turn lamp per se. Instead, the normal green lamp flashes rapidly, indicating permission to go straight as well as make a left turn in front of opposing traffic, which is being held by a steady red lamp. (This "advance green," or flashing green can be somewhat startling and confusing to drivers not familiar with this system. This also can cause confusion amongst visitors to British Columbia, where a flashing green signal denotes a pedestrian-controlled crosswalk. For this reason, Ontario is phasing out the use of flashing green signals and instead replacing them with arrows.)[50][51]

Flashing yellow arrows typically indicate that road users must give way (to other drivers and pedestrians) before making a movement in the direction of the arrow. These are used because they are safer, cause less delay, and are more flexible. Flashing yellow arrows will normally be located below the solid yellow.[52]

Without an all-red phase, cross-turning traffic may be caught in a yellow trap. When the signal turns yellow, a turning driver may assume oncoming traffic will stop and a crash may result. For this reason, the US bans sequences that may cause a yellow trap.[53] This can also happen when emergency vehicles or railroads preempt normal signal operation. [54] In the United States, signs reading "Oncoming traffic has extended green" or "Oncoming traffic may have extended green" must be posted at intersections where the "yellow trap" condition exists.[55][56]

A hook turn, or two-stage turn, is generally used to allow lighter vehicles such as cycles or motorcycles to turn across oncoming traffic. Such vehicles when permitted can move to the nearside of the junction and position themselves in front of side road traffic, waiting for the side road traffic to gain a green signal before completing the turn. In Australia, such turns were the standard right turn until the 1950s.[57] Hook turns remaining in Australia are generally in the Melbourne central city area to ensure cars do not wait on centre-lane tram tracks and delay trams.

An example of an LED traffic light in Australia

Traditionally, incandescent and halogen bulbs were used. Because of the low efficiency of light output and a single point of failure (filament burnout), some traffic authorities are choosing to retrofit traffic signals with LED arrays that consume less power, have increased light output, and last significantly longer. Moreover, in the event of an individual LED failure, the aspect will still operate albeit with a reduced light output. The light pattern of an LED array can be comparable to the pattern of an incandescent or halogen bulb fitted with a prismatic lens.

The low energy consumption of LED lights can pose a driving risk in some areas during winter. Unlike incandescent and halogen bulbs, which generally get hot enough to melt away any snow that may settle on individual lights, LED displays – using only a fraction of the energy – remain too cool for this to happen.[59][60] As a response to the safety concerns, a heating element on the lens was developed.[61][62]

Traffic signals installed in Shelton, Washington, seen off-axis from the intended viewing area (top) and from the signal's intended viewing area (bottom).

From off-axis, these signals appear to be "off" or invisible to adjacent lanes of traffic during the daytime. Only a faint glow can be seen when viewed at night.

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

Using a special aluminium foil-based adhesive tape, these signals are "masked" or programmed by the programming lens so that only certain lanes of traffic will view the indication. At the front of these programmable visibility signals is a 12" Fresnel lens, each lens tinted to meet United States Institute of Transportation Engineers (ITE) chromaticity and luminance standards. The Fresnel lens collimates the light output created by the lamp and creates a uniform display of light for the lane in which it is intended. These signals were first developed by the 3M Company in the late 1960s and were 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 misinterpretation from other lanes. For example, a Fresnel lens on an adjacent through-lane signal may be aimed to prevent left-turning traffic from anticipating its own green arrow.

Today, McCain Traffic Systems is the only U.S.-based manufacturer producing optically programmable traffic signals similar to the 3M model. Intelight Inc. manufactures a programmable traffic signal that uses a software-controlled LED array and electronics to steer the light beam toward the desired approach.[63] The signal is programmed unlike the 3M and McCain models. It requires a connection to a laptop or smartphone with the manufacturer’s software installed. Connections can be made directly with a direct-serial interface kit, or wirelessly with a radio kit over WIFI to the signal.

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.

An animated GIF shows a traffic light in 3 phases.

Conventional traffic signal lighting, still common in some areas, uses a standard light bulb. Typically, a 67, 69, or 115-watt medium-base (household lamp in the US) light bulb provides the illumination. The 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 used a beaded prismatic lens with a "smiley" pattern embossed into the bottom of each lens.

In the United States, traffic lights are currently designed with lights approximately 12 inches (300 mm) in diameter. Previously the standard had been 8 inches (200 mm); however, those are slowly being phased out in favour 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 inches (200 mm) 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. However, following a study sponsored by the UK Highways Agency and completed by Aston University, Birmingham, UK, an enhanced optical design was introduced in the mid-1990s. Criticism of sunlight washout (cannot see the illuminated signal due to sunlight falling on it), and sun-phantom (signal appearing to be illuminated even when not due to sunlight reflecting from the parabolic mirror at low sun angles), led to the design of a signal that used lenslets to focus light from a traditional incandescent bulb through apertures in a matt black front mask. This cured both problems in an easily manufactured solution. This design proved very successful and was taken into production by a number of traffic signal manufacturers through the engineering designs of Dr. Mark Aston, working firstly at the SIRA Ltd in Kent, and latterly as an independent optical designer. The manufacturers took a licence for the generic design from the Highways Agency, with Dr. Aston engineering a unique solution for each manufacturer. Producing both bulb and LED versions of the signal aspects, these signals are still the most common type of traffic light on UK roads. With the invention of anti-phantom, highly visible Aston lenses, lights of 8 inches (200 mm) could be designed to give the same output as plain lenses, so a larger surface area was unnecessary. Consequently, lights of 12 inches (300 mm) are no longer approved for use in the UK and all lights installed on new installations have to be 200 mm (8 in) in accordance with TSRGD (Traffic Signs Regulations and General Directions). Exemptions are made for temporary or replacement signals.

Temporary traffic light near Hazlerigg, England

With technologies in developed countries continuing to advance, there is now an increasing move to develop and implement smart traffic lights on the roads. These are systems that adapt to information that is received from a central computer about the position, speed, and direction of vehicles. They try to communicate with cars to alert drivers of impending light changes and reduce motorists' waiting time considerably. Trials are currently being conducted for the implementation of these advanced traffic lights but there are still many hurdles to widespread use that need to be addressed; one of which is the fact that few cars yet have the required systems to communicate with these lights.

In some instances, small vehicles such as motorcycles, scooters, and bicycles may not be detected, leaving the traffic light controller unaware of their presence. When this occurs, the small vehicle may fail to receive the right of way when the traffic light controller skips their phase, such as at traffic lights that are programmed to remain green for the main street and to only service minor movements, such as the side street or the main street left turn lane, on an as-needed basis when there is demand.

For example, small vehicles may not be detected by an induction loop sensor, such as one whose sensitivity has been set too high in an attempt to avoid false triggers. While a typical motor vehicle has a sufficient mass of metal such as steel that interacts with the sensor's magnetic field,[64] motorcycles and scooters have much less mass than cars, and bicycles may not even be constructed with metal. This situation most often occurs at 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.[65]

Drivers spend on average around 2% of journey time passing through signalised junctions.[68]

Traffic lights can increase the traffic capacity at intersections and reduce delay for side road traffic, but can also result in increased delay for main road traffic.[69] Hans Monderman, the innovative Dutch traffic engineer, and pioneer of shared space schemes, was sceptical of their role, and is quoted as having said of them: "We only want traffic lights where they are useful and I haven't found anywhere where they are useful yet."[70]

A World Economic Forum study found that signalised junctions are linked to higher rates of localised air pollution. Drivers accelerate and stop frequently at lights and as such peak particle concentration can be around 29 times higher than during free-flow conditions. The WEF recommends that traffic authorities synchronise traffic signals, consider alternative traffic management systems and consider placing traffic lights away from residential areas, schools, and hospitals.[68]

The separation of conflicting streams of traffic in time can reduce the chances of right-angle collisions by turning traffic and cross traffic, but they can increase the frequency of rear-end crashes by up to 50%.[71] Since right-angled and turn-against-traffic collisions are more likely to result in injuries, this is often an acceptable trade-off. They can also adversely affect the safety of bicycle and pedestrian traffic.

Between 1979 and 1988, the city of Philadelphia, Pennsylvania, removed signals at 199 intersections that were not warranted. On average, the intersections had 24% fewer crashes after the unwarranted signals were removed.[71] The traffic lights had been erected in the 1960s because of since-resolved protests over traffic. By 1992, over 800 traffic lights had been removed at 426 intersections, and the number of crashes at these intersections dropped by 60%.[72]

In the United States, the criteria for installation of a traffic control signal are prescribed by the Manual on Uniform Traffic Control Devices (MUTCD), which defines the criteria in nine warrants:[73]

• Eight-hour vehicular volume. Traffic volume must exceed prescribed minima for eight hours of an average weekday.
• Four-hour vehicular volume. Traffic volume must exceed prescribed minima for four hours of an average weekday.
• Peak hour volume or delay. This is applied only in unusual cases, such as office parks, industrial complexes, and park and ride lots that attract or discharge large numbers of vehicles in a short time, and for a minimum of one hour of an average weekday. The side road traffic suffers undue delays when entering or crossing the major street.
• Pedestrian volume. If the traffic volume on a major street is so heavy that pedestrians experience excessive delays 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 is usually required to 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, it only suggests that they may be suitable. It could be that a roundabout would work better. There may be other unconsidered conditions that lead traffic engineers to conclude that a signal is undesirable. For example, it may be decided 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.

In some jurisdictions (such as New York City), there are ordinances or by-laws against "gridlocking". A motorist entering an intersection (even if on a green light) but unable to proceed and who gets stranded in the intersection (when traffic ahead fails to proceed), and who remains after the light turns red (thus blocking traffic from other directions) may be cited. The definition of the intersection area is a 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 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.

A driver comes to a stop on the crosswalk beyond the stop line as a result of attempting to avoid a red light violation.

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

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

• by police officers observing traffic, 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

Traffic light in Chelyabinsk, Russia showing a green light with a digital countdown (in the centre aspect) of the time left until the signal switches to amber

Red light running occurs when a motorist enters a junction any time after the signal light turns red unless a legal turn on red manoeuvre is completed.[74] A US national survey in 2019 found that 86% of drivers thought it was very dangerous to run a red light at speed, but 31% reported that they had done so in the past 30 days.[75]

Common reasons for red light running include inattentive driving, following an oversized vehicle, or during inclement weather. Drivers are more likely to run red lights during peak traffic hours or if the signal is on a downward slope.[76]Uber had to apologise in 2016 after a self-driving Uber vehicle ran a red light "due to human error" in San Francisco.[77] Red light runners involved in multiple-vehicle crashes are more likely male, younger, involved in prior crashes, or have alcohol-impaired driving convictions.[78] According to the IIHS Insurance Institute for Highway Safety, every year red light running causes hundreds of deaths, thousands of injuries, and hundreds of millions of dollars in related costs.[78]

In Spring 2015, Hunter College in New York City completed an observational study of red light running, the first of its kind. The conclusion, after monitoring 3,259 vehicles at 50 intersections over a period of days around the 5 boroughs, found that almost 10% of vehicles and 15% of taxis ran the red light, amounting to nearly 400,000 red lights run every single day.[79]

Source: IIHS[80]

Red light running can be deterred through enforcement or engineering strategies. It is normally illegal and may incur a fine, demerit points, or a stricter sentence. As of 2021, Norway has the highest fine in Europe for red light running at €756 equivalent.[81] The US Federal Highway Administration recommends that engineers:[82]

• improve signal visibility (better signal placement, increased signal size or improved line of sight);
• improve signal conspicuity (to better capture the motorist's attention);
• increase the likelihood of stopping (with signal head signs, advance-warning flashers or rumble strips); and
• address intentional violations (through signal optimisation, properly timed yellow-change intervals, and all-red clearance intervals).

After the Hunter College study, New York City Mayor Bill de Blasio launched a Vision Zero plan to reduce vehicular and pedestrian fatalities. The New York City Police Department's 2014 TrafficStat report showed 33,577 red light tickets for 2013, which is a 126% increase in the number of failure-to-yield summonses and red-light running violations.[83]

In some countries, red light cameras are used for either the driver or the vehicle's owner. An automated camera is connected to the triggering mechanism for the corresponding traffic light, which is programmed to photograph a vehicle and driver crossing against the light. Either the driver or the vehicle's owner (depending on the locale) are fined for the violation. In some jurisdictions, including the United States and Italy, private companies have been contracted to operate traffic-related cameras and 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.[84][85] Despite the fact that cameras can reduce the number of crashes, it has been proven that at these intersections drivers tended to react quicker to an amber light change when stopping.[86][87] The consequence of this change could be a slight decline in the intersection capacity.

Red light cameras in New South Wales, Australia, are activated only if a motorist enters an intersection 0.3 seconds after the light has turned red.[88]

In some jurisdictions, yellow light running may also be illegal, unless a driver cannot stop before the stop line safely. For example, this rule is in place in states within Australia,[89] Russia (which also has a flashing green phase and possibly countdown timers to warn drivers of a yellow phase),[90] and the UK.[91]

The Amber Gambler Twins is one of many public interest films trying to change the public's behaviour regarding the running of yellow lights.[92][93]

An intersection with blue confirmation lights in Newport News, Virginia

Another way police officers have begun to combat red light runners is with blue or white Confirmation Lights.[94] These lights can be seen from any angle in an intersection and are typically utilised by emergency responders who actuate traffic signal preemption devices to verify that other motorists are facing a red signal. However, confirmation lights also assist officers – who do not have to have a line of sight with a red light – to catch vehicles illegally entering an intersection. They are only lit when the red light on the same signal head is on. Some intersections will also have multiple confirmation lights for a single direction of travel if there are different signals for different directions. These lights are separate from the main ones, often protruding above or below the main traffic light, and are much smaller than a standard light to help avoid confusion.

In the Netherlands, many red traffic signals can be seen from the side via a small bulbous window, indicating to drivers (and police officers) whether the signal in the crossing direction is actually red or not, by simply leaking out some of the red light through the side of the traffic signal. This has gradually become less common as traditional incandescent signals are replaced by LED signals, while increasingly red light cameras are used to detect driving-through-red violations.

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-away. Green would indicate racing is underway, while amber would indicate to slow or while following a pace car; red would indicate to stop, probably for emergency reasons.

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

Another type of traffic light that is used in racing is the Christmas Tree, which is used in drag racing. The Christmas Tree has six lights: a blue staging light, three amber lights, a green light, and a red light. The blue staging light is divided into two parts: Pre-stage and stage. Sometimes, there are two sets of bulbs on top of each other to represent them. Once a driver is staged at the starting line, then the starter will activate the light to commence racing, which can be done in two ways. If a Pro tree is used, then the three amber lights will flash at the same time. For the Sportsman tree, the amber light will flash from top to bottom. When the green light comes up, the race officially begins but if a driver crosses the line before that happens, then a red light will come up and that will be a foul.

The colours red, yellow, 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 colours, with red and green at the extremes.[96]