Traffic light control and coordination
Encyclopedia
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. A variety of different control systems are used to accomplish this, ranging from simple clockwork mechanisms to sophisticated computerized control and coordination systems that self-adjust to minimize delay to people using the road.
inside a cabinet mounted on a concrete
pad.
Although some electro-mechanical controllers are still in use (New York City
still has 4,800), modern traffic controllers are solid state. The cabinet typically contains a power panel, to distribute electrical power in the cabinet; a detector interface panel, to connect to loop detectors and other detectors; detector amplifiers; the controller itself; a conflict monitor unit; flash transfer relays; a police panel, to allow the police to disable the signal; and other components.
In the United States, controllers are standardized by the NEMA
, which sets standards for connectors, operating limits, and intervals. The TS-1 standard was introduced in 1976 for the first generation of solid-state controllers.
Traffic controllers use the concept of phases, which are directions of movement grouped together. For instance, a simple intersection may have two phases: North/South, and East/West. A 4-way intersection with independent control for each direction and each left-turn, will have eight phases. Controllers also use rings; each ring is an array of independent timing sequences. For example, with a dual-ring controller, opposing left-turn arrows may turn red independently, depending on the amount of traffic. Thus, a typical controller is an 8-phase, dual ring control.
Solid state controllers are required to have an independent conflict monitor unit (CMU), which ensures fail-safe
operation. The CMU monitors the outputs of the controller, and if a fault is detected, the CMU uses the flash transfer relays to put the intersection to FLASH, with all red lights flashing, rather than displaying a potentially hazardous combination of signals. The CMU is programmed with the allowable combinations of lights, and will detect if the controller gives conflicting directions a green signal, for instance.
In the late 1990s, a national standardization effort known as the Advanced transportation controller
(ATC) was undertaken in the United States by the Institute of Transportation Engineers
. The project attempts to create a single national standard for traffic light controllers. The standardization effort is part of the National Intelligent transportation system
program funded by various highway bills, starting with ISTEA in 1991, followed by TEA-21, and subsequent bills. The controllers will communicate using National Transportation Communications for ITS Protocol (NTCIP), based on Internet Protocol
, ISO/OSI, and ASN.1.
Traffic lights must be instructed when to change phase and they are usually coordinated so that the phase changes occur in some relationship to other nearby signals or to the press of a pedestrian button or to the action of a timer or a number of other inputs.
of an intersection would benefit from being adapted to the dominant flows changing over the time of the day.
The controller uses input from detectors, which are sensors that inform the controller processor whether vehicles or other road users are present, to adjust signal timing and phasing within the limits set by the controllers programming. It can give more time to an intersection approach that is experiencing heavy traffic, or shorten or even skip a phase that has little or no traffic waiting for a green light. Detectors can be grouped into three classes: in-pavement detectors, non-intrusive detectors, and detection for non-motorized road users.
s buried in the road to detect the presence of traffic waiting at the light, and thus can reduce the time when a green signal is given to an empty road. A timer is frequently used as a default during times of very low traffic density and as a backup in case the sensors fail. The sensor loops typically work in the same fashion as metal detector
s. Consequently small vehicles or those with low metal content may fail to be detected causing them to wait indefinitely unless there is also a default timer as part of the control system.
waves, or acoustic sensors to detect the presence of vehicles at the intersection waiting for right of way. These over-roadway sensors are more favorable than in-roadway sensors because they are immune to the natural degradation associated with paved right-of-way, competitively priced to install in terms of monetary and labour cost and danger to installation personnel, and have the capacity to act as real-time traffic management devices, act as multi-lane detectors, and collect data types not available from in-roadway sensors
Non-motorized users are classified as pedestrians, bicyclists, and equestrians. Provisions for detecting these users include demand buttons and tuned detectors.
Some traffic lights at pedestrian crossings, especially those away from junctions, include a button which must be pressed in order to activate the timing system. This is generally accompanied by a large display reading "wait", which lights up when the button is pressed, and off when the lights enter the red phase. Often, other displays, such as countdowns or the green & red pedestrian lights are included in this panel. With the advent of computer-controlled traffic lights, in many countries activation buttons have become obsolete. In fact, most in New York City have been disconnected. Conversely, new installations of activation buttons increasingly provide for specific user groups, including audible buttons and signals for visually impaired users and so-called Pegasus crossing
buttons for users on horseback.
Standard signal detectors have a hard time detecting bicyclists, because of the low metal content of typical bicyclists. If a bicyclist rides directly over the wires of a detector loop, it may detect the cyclist. However, it does not always work, and few cyclists know to do it. At locations where cyclists are common, a special detector loop tuned for cyclists may be used. A small bicycle symbol is often marked on the pavement to inform the cyclist where to stop in order to actuate the signal. Other places simply place an additional pedestrian button near the curb where a cyclist can reach it.
, France
and The Netherlands), this "green band" system is used to limit speeds in certain areas. Lights are timed in such a way that motorists can drive through without stopping if their speed is lower than a given limit, mostly 50 km/h (30 mph) in urban areas. This system is known as "grüne Welle" in German, "vague verte" in French, or "groene golf" in Dutch (English: "green wave
"). Such systems were commonly used in urban areas of the United States since the 1940s, but are less common today.
In modern coordinated signal systems, it is possible for drivers to travel long distances without encountering a red light. This coordination is done easily only on one-way streets with fairly constant levels of traffic. Two-way streets are often arranged to correspond with rush hour
s to speed the heavier volume direction. Congestion can often throw off any coordination, however. On the other hand, some traffic signals are coordinated to prevent drivers from encountering a long string of green lights. This practice discourages high volumes of traffic by inducing delay yet preventing congestion. Speed is self-regulated in coordinated signal systems; drivers traveling too fast will arrive on a red indication and end up stopping, drivers traveling too slowly will not arrive at the next signal in time to utilize the green indication. In synchronized systems, however, drivers will often use excessive speed in order to "make" as many lights as possible.
More recently even more sophisticated methods have been employed. Traffic lights are sometimes centrally controlled by monitors or by computers to allow them to be coordinated in real time
to deal with changing traffic patterns. Video camera
s, or sensor
s buried in the pavement can be used to monitor traffic patterns across a city. Non-coordinated sensors occasionally impede traffic by detecting a lull and turning red just as cars arrive from the previous light. The most high-end systems use dozens of sensors and cost hundreds of thousands of dollar
s per intersection, but can very finely control traffic levels. This relieves the need for other measures (like new roads) which are even more expensive.
Benefits include:
Examples:
Traffic controller systems
A traffic signal is typically controlled by a controllerController (control theory)
In control theory, a controller is a device which monitors and affects the operational conditions of a given dynamical system. The operational conditions are typically referred to as output variables of the system which can be affected by adjusting certain input variables...
inside a cabinet mounted on a concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
pad.
Although some electro-mechanical controllers are still in use (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...
still has 4,800), modern traffic controllers are solid state. The cabinet typically contains a power panel, to distribute electrical power in the cabinet; a detector interface panel, to connect to loop detectors and other detectors; detector amplifiers; the controller itself; a conflict monitor unit; flash transfer relays; a police panel, to allow the police to disable the signal; and other components.
Néma
Néma is a town in southeastern Mauritania, close to the border with Mali. It is located at around . It is the capital of Hodh Ech Chargui Region and of the Néma Department....
, which sets standards for connectors, operating limits, and intervals. The TS-1 standard was introduced in 1976 for the first generation of solid-state controllers.
Traffic controllers use the concept of phases, which are directions of movement grouped together. For instance, a simple intersection may have two phases: North/South, and East/West. A 4-way intersection with independent control for each direction and each left-turn, will have eight phases. Controllers also use rings; each ring is an array of independent timing sequences. For example, with a dual-ring controller, opposing left-turn arrows may turn red independently, depending on the amount of traffic. Thus, a typical controller is an 8-phase, dual ring control.
Solid state controllers are required to have an independent conflict monitor unit (CMU), which ensures fail-safe
Fail-safe
A fail-safe or fail-secure device is one that, in the event of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or danger to personnel....
operation. The CMU monitors the outputs of the controller, and if a fault is detected, the CMU uses the flash transfer relays to put the intersection to FLASH, with all red lights flashing, rather than displaying a potentially hazardous combination of signals. The CMU is programmed with the allowable combinations of lights, and will detect if the controller gives conflicting directions a green signal, for instance.
In the late 1990s, a national standardization effort known as the Advanced transportation controller
Advanced transportation controller
The advanced transportation controller is a standardization effort being undertaken by the United States Department of Transportation as part of their Intelligent transportation system efforts. The ATC is being developed to provide an open platform for hardware and software for a wide variety of...
(ATC) was undertaken in the United States by the 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...
. The project attempts to create a single national standard for traffic light controllers. The standardization effort is part of the National Intelligent transportation system
Intelligent transportation system
The term intelligent transportation systems refers to information and communication technology that improve transport outcomes such as transport safety, transport productivity, travel reliability, informed travel choices, social equity, environmental performance and network operation...
program funded by various highway bills, starting with ISTEA in 1991, followed by TEA-21, and subsequent bills. The controllers will communicate using National Transportation Communications for ITS Protocol (NTCIP), based on Internet Protocol
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
, ISO/OSI, and ASN.1.
Traffic lights must be instructed when to change phase and they are usually coordinated so that the phase changes occur in some relationship to other nearby signals or to the press of a pedestrian button or to the action of a timer or a number of other inputs.
Fixed time control
The simplest control system uses a timer (fixed-time): each phase of the signal lasts for a specific duration before the next phase occurs; this pattern repeats itself regardless of traffic. Many older traffic light installations still use these, and timer-based signals are effective in one way grids where it is often possible to coordinate the traffic lights to the posted speed limit. They are however disadvantageous when the signal timingSignal timing
Signal timing is the technique which traffic engineers use to determine who has the right-of-way at an intersection. Signal timing involves deciding how much green time the traffic lights shall provide at an intersection approach, how long the pedestrian WALK signal should be, and many numerous...
of an intersection would benefit from being adapted to the dominant flows changing over the time of the day.
Dynamic control
Dynamic, or actuated, signals are programmed to adjust their timing and phasing to meet changing traffic conditions. The system adjusts signal phasing and timing to minimize the delay of people going through the intersection. It is also commonplace to alter the control strategy of a traffic light based on the time of day and day of the week, or for other special circumstances such as a major event causing unusual demand at an intersection.The controller uses input from detectors, which are sensors that inform the controller processor whether vehicles or other road users are present, to adjust signal timing and phasing within the limits set by the controllers programming. It can give more time to an intersection approach that is experiencing heavy traffic, or shorten or even skip a phase that has little or no traffic waiting for a green light. Detectors can be grouped into three classes: in-pavement detectors, non-intrusive detectors, and detection for non-motorized road users.
In-pavement detectors
These detectors are buried in or under the roadway. Inductive Detector loops are the most common type. They are sensorSensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s buried in the road to detect the presence of traffic waiting at the light, and thus can reduce the time when a green signal is given to an empty road. A timer is frequently used as a default during times of very low traffic density and as a backup in case the sensors fail. The sensor loops typically work in the same fashion as metal detector
Metal detector
A metal detector is a device which responds to metal that may not be readily apparent.The simplest form of a metal detector consists of an oscillator producing an alternating current that passes through a coil producing an alternating magnetic field...
s. Consequently small vehicles or those with low metal content may fail to be detected causing them to wait indefinitely unless there is also a default timer as part of the control system.
Non-intrusive detectors
It is sometimes more advantageous and cost effective to install over-roadway sensors than cutting the road and embedding inductive loops. These technologies include video image processors, sensors that use EMElectromagnetism
Electromagnetism is one of the four fundamental interactions in nature. The other three are the strong interaction, the weak interaction and gravitation...
waves, or acoustic sensors to detect the presence of vehicles at the intersection waiting for right of way. These over-roadway sensors are more favorable than in-roadway sensors because they are immune to the natural degradation associated with paved right-of-way, competitively priced to install in terms of monetary and labour cost and danger to installation personnel, and have the capacity to act as real-time traffic management devices, act as multi-lane detectors, and collect data types not available from in-roadway sensors
Non-motorized user detection
Some traffic lights at pedestrian crossings, especially those away from junctions, include a button which must be pressed in order to activate the timing system. This is generally accompanied by a large display reading "wait", which lights up when the button is pressed, and off when the lights enter the red phase. Often, other displays, such as countdowns or the green & red pedestrian lights are included in this panel. With the advent of computer-controlled traffic lights, in many countries activation buttons have become obsolete. In fact, most in New York City have been disconnected. Conversely, new installations of activation buttons increasingly provide for specific user groups, including audible buttons and signals for visually impaired users and so-called Pegasus crossing
Pegasus crossing
A Pegasus crossing is a type of signalised pedestrian crossing with special consideration for horse riders. This type of crossing is nicknamed after the mythical winged horse, Pegasus...
buttons for users on horseback.
Standard signal detectors have a hard time detecting bicyclists, because of the low metal content of typical bicyclists. If a bicyclist rides directly over the wires of a detector loop, it may detect the cyclist. However, it does not always work, and few cyclists know to do it. At locations where cyclists are common, a special detector loop tuned for cyclists may be used. A small bicycle symbol is often marked on the pavement to inform the cyclist where to stop in order to actuate the signal. Other places simply place an additional pedestrian button near the curb where a cyclist can reach it.
Coordinated control
Attempts are often made to place traffic signals on a coordinated system so that drivers encounter long strings of green lights. The distinction between coordinated signals and synchronized signals is very important. Synchronized signals all change at the same time and are only used in special instances or in older systems. Coordinated systems are controlled from a master controller and are set up so lights "cascade" in sequence so platoons of vehicles can proceed through a continuous series of green lights. A graphical representation of phase state on a two-axis plane of distance versus time clearly shows a "green band" that has been established based on signalized intersection spacing and expected vehicle speeds. In some countries (e.g. GermanyGermany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
, France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
and The Netherlands), this "green band" system is used to limit speeds in certain areas. Lights are timed in such a way that motorists can drive through without stopping if their speed is lower than a given limit, mostly 50 km/h (30 mph) in urban areas. This system is known as "grüne Welle" in German, "vague verte" in French, or "groene golf" in Dutch (English: "green wave
Green wave
A green wave is an intentionally induced phenomenon in which a series of traffic lights are coordinated to allow continuous traffic flow over several intersections in one main direction....
"). Such systems were commonly used in urban areas of the United States since the 1940s, but are less common today.
In modern coordinated signal systems, it is possible for drivers to travel long distances without encountering a red light. This coordination is done easily only on one-way streets with fairly constant levels of traffic. Two-way streets are often arranged to correspond with rush hour
Rush hour
A rush hour or peak hour is a part of the day during which traffic congestion on roads and crowding on public transport is at its highest. Normally, this happens twice a day—once in the morning and once in the evening, the times during when the most people commute...
s to speed the heavier volume direction. Congestion can often throw off any coordination, however. On the other hand, some traffic signals are coordinated to prevent drivers from encountering a long string of green lights. This practice discourages high volumes of traffic by inducing delay yet preventing congestion. Speed is self-regulated in coordinated signal systems; drivers traveling too fast will arrive on a red indication and end up stopping, drivers traveling too slowly will not arrive at the next signal in time to utilize the green indication. In synchronized systems, however, drivers will often use excessive speed in order to "make" as many lights as possible.
Real-time computing
In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...
to deal with changing traffic patterns. Video camera
Video camera
A video camera is a camera used for electronic motion picture acquisition, initially developed by the television industry but now common in other applications as well. The earliest video cameras were those of John Logie Baird, based on the electromechanical Nipkow disk and used by the BBC in...
s, or sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s buried in the pavement can be used to monitor traffic patterns across a city. Non-coordinated sensors occasionally impede traffic by detecting a lull and turning red just as cars arrive from the previous light. The most high-end systems use dozens of sensors and cost hundreds of thousands of dollar
Dollar
The dollar is the name of the official currency of many countries, including Australia, Belize, Canada, Ecuador, El Salvador, Hong Kong, New Zealand, Singapore, Taiwan, and the United States.-Etymology:...
s per intersection, but can very finely control traffic levels. This relieves the need for other measures (like new roads) which are even more expensive.
Benefits include:
- Increasing the traffic handling capacity of roads.
- Reduce accidents, both vehicular and pedestrian. Encourages travel within the speed limit to meet green lights.
- Reduce unnecessary stopping and starting of traffic - this in turn reduces fuel consumption, air pollution, noise and vehicle wear and tear.
- Improve journey time.
- Reduce driver frustration and 'road rage'.
Examples:
- New York CityNew York CityNew 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...
: 6,000 (of a total of 10,800) signalized intersections are controlled by a central computer - TorontoTorontoToronto 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...
: 83% of its signals are controlled by the Main Traffic Signal System (MTSS). 15% also use the SCOOT (Split Cycle and Offset Optimization Technique), an adaptive signal control system. - SydneySydneySydney is the most populous city in Australia and the state capital of New South Wales. Sydney is located on Australia's south-east coast of the Tasman Sea. As of June 2010, the greater metropolitan area had an approximate population of 4.6 million people...
: 3,400 traffic signals co-ordinated by the Sydney Co-ordinated Adaptive Traffic System (SCATS). Designed and developed by RTA, the system was first introduced in 1963 and progressively developed since then. By October 2010, SCATS was licensed to 33,200 intersections in 144 cities across 24 countries worldwide, including Singapore, Hong Kong, Dublin, Tehran and Minneapolis and Detroit. - MelbourneMelbourneMelbourne 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...
: 3,200 traffic lights across Victoria, including regional areas such as Geelong and Ballarat, using SCATS. Some 500 intersections also have tram and bus priority. - AdelaideAdelaideAdelaide is the capital city of South Australia and the fifth-largest city in Australia. Adelaide has an estimated population of more than 1.2 million...
: 580 sets of coordinated traffic lights throughout the metropolitan region managed by the Adelaide Coordinated Traffic Signal (ACTS) System.
Other types of control
- Failures: If power is still available, a flashing amber light is used to warn of an intersection. Methods to distinguish the main road from the secondary road (and hence right of way) include using yield (give way) signs, stop signs or a flashing red light on the secondary road as well as written signage. In some countries including Australia, the road rules outline procedures such as giving way to the right.
- Part-time operation: Some traffic lights will not operate at night or when traffic is very light. Some may only operate at particular set times (e.g. during working hours of a major factory) or only during special events such as sports or exhibitions. When not operating, the same measures as with failures are used. Part-time operation has advantages and disadvantages.
- Railroad preemption: Traffic signals are activatated to coincide with the approach of a train, often where the intersection is near a rail crossing. See also Railroad preemption
- Bus and Transport Prority: Traffic signals are activatated to coincide with the arrival of a bus or tram along a busway, bus lane or tramway. See also Bus priorityBus priorityBus priority or transit signal priority is a name for various techniques to speed up bus public transport services at intersections with traffic signals amongst other methods. Trams and light rail vehicles can also be given priority...
- Emergency Vehicles Some lights outside of fireFire stationA fire station is a structure or other area set aside for storage of firefighting apparatus , personal protective equipment, fire hose, fire extinguishers, and other fire extinguishing equipment...
or rescue stations have no green, as they may turn only amber and then red when fire trucksFire apparatusA 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...
, ambulances, or other emergency vehicles of the like are exiting the station en route to an emergency. See also Traffic signal preemptionTraffic signal preemptionTraffic signal preemption is a type of system that allows the normal operation of traffic lights to be preempted, often to assist emergency vehicles...