Traffic lights have also been used in pieces of artwork, particularly the Traffic Light tree 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, a Formula One 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 However, the system was withdrawn after the 2008 Singapore Grand Prix, due to the fact that it heavily delayed Felipe Massa during the race, when he was in the lead.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.

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.

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 itself; this intervening period is called the all-red time. A typical all-red time is 2 sec 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 4 seconds, allowing drivers who could not stop at the amber light enough time to clear the intersection without causing a collision.

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

Jurisdictions differ somewhat on how to deal with “red light running” — attempts by motorists to race to an intersection while facing a yellow light, in an attempt to beat the red. In some locales, as long as the light is yellow when the motorist enters the intersection, no offense has been committed; in others, if the light turns red at any time before the motorist clears the intersection, then an offense occurs. In Oregon and other places, a stricter standard applies-—running an amber light is an offense, unless the motorist is unable to stop safely. This standard has been criticized as ambiguous and difficult to enforce (red light cameras in Oregon are activated only if a motorist enters the intersection on a red). Red light cameras in NSW, Australia are activated only if a motorist enters an intersection 0.3 seconds after the light has turned red.

In some jurisdictions (such as Toronto, Washington D.C., New York City), and the state of California, there are ordinances against “gridlocking.” Any motorist who enters an intersection (even if on a green light) but does not ensure that one can proceed through the intersection and gets stuck in the middle of the intersection (when traffic ahead fails to proceed), and remains there after the light turns red (thus blocking traffic coming from other directions) may receive a citation. 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.

Some local driving traditions may be legally questionable. A prime example is the Pittsburgh left. Although failing to yield to oncoming traffic while navigating a turn is a serious traffic violation and is prohibited in the Commonwealth of Pennsylvania, many drivers will make a Pittsburgh left. The Pittsburgh Left: a driver waiting at a red light to turn left is allowed to turn by oncoming traffic when the light turns green, instead of yielding the right-of-way as the law requires. Although illegal, such behavior is regarded as not only customary but also polite in the city of Pittsburgh.

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

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

• By police officers observing traffic through the light, and pulling over and issuing citations to motorists who violate the signal.
• As a result of an accident investigation, if it is determined that one or more motorists ran the red light — even if the incident was not observed by a police officer.
• With red light cameras.

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.

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

In the US MUTCD, there are eight warrants for traffic signals:

• Eight-Hour Vehicular Volume. Traffic volume must exceed prescribed minima for eight hours of each weekday.
• Four-Hour Vehicular Volume. Traffic volume must exceed prescribed minima for four hours of each weekday.
• Peak Hour Volume or Delay. This is applied only in unusual cases, such as office parks, industrial complexes, or 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 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 there not enough safe gaps in traffic for school 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.

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 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.

While the implications of this occurrence appear to be trivial, there are some important consequences that follow. Safety is a concern that arises when a motorist eventually has no other choice than to pass through the intersection while the light is red, or perform a U turn in a scenario where doing so would be risky. People who encounter this on a daily basis they may be discouraged from operating smaller more fuel efficient vehicles.

Japan, South Korea, Taiwan, Thailand and Croatia, along with some jurisdictions in the U.S. (including all in the State of New Mexico),some places in Canada and Mexico mount lights with their multiple faces arranged horizontally, often with supplemental vertical signals on the side, while others use vertical signals almost exclusively. Horizontal signals have consistent orientation, like their vertical counterparts.^[19] 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 traffic light position is very common in Houston on most intersections.^{[citation needed]}

California is particularly rigorous in ensuring that drivers can see the current state of a traffic light. One entrance to a typical large intersection, with three through lanes, two dedicated left-turn lanes, and a crosswalk, may have as many as three traffic lights for the left-turn lanes, three for the through lanes, and a pedestrian signal for the crosswalk. Those numbers must be multiplied by four to cover all four ways to enter a typical intersection.

In addition to being positioned and mounted for desired visibility for their respective traffic, some traffic lights are also aimed, louvered, or shaded to minimize mis-interpretation from other lanes. For example, a Fresnel lens on an adjacent through-lane signal may be aimed to prevent left-turning traffic from anticipating its own green arrow. One example of the Fresnel lens application common in the USA is the 3M Model 131 traffic signal (dubbed the “Programmed Visibility” signal). Although 3M has recently discontinued the M-131, it became a popular traffic signal for skewed or complex intersections. Today, McCain Traffic Systems (a traffic signal manufacturing company based in Vista, California) 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.

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. The stop line alignment is done to prevent vehicles blocking any crosswalk blocking and allow for better pedestrian traffic flow. There may also be a special area a few metres in advance of the stop line where cyclists may legally wait but not motor vehicles; this advanced stop line 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.

In Lloydminster, a city straddling the provincial border between Alberta and Saskatchewan, an unusual jurisdictional arrangement can be found: motorists proceeding east at the intersection situated on the border must, while in Alberta, stop for a red signal located in Saskatchewan; the reverse applies for westbound motorists. If such a motorist were to disregard the signal and collide with a vehicle proceeding from that motorist’s left, the collision would occur in a province other than the one where the traffic control device is located.

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 (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 millimetres (8 in) in accordance with TSRGD (Traffic Signs Regulations and General Directions). Exemptions are made for temporary or replacement signals.

In the state of Victoria, Australia, lights used to have long visors with a slanted cut on the side nearest the road (for approach side) and none for the opposite side. These lights also had a sort of horizontal filter covering the upper half of the visor, except on arrows. This design eliminated or minimised washout of the light by a low-lying sun. This type of design has yet to surface outside of Victoria. LED-based lights, however, do not follow this standard, adopting the national standard of having a short visor and cut on both sides, although the cut is also applied to the lights on the opposite side of the junction. Some installations do not have cuts on the opposite side. There exists at least 1 LED-based light using the visor and filter set of its predecessor.