Traffic light control and coordination
A traffic signal is typically controlled by a controller inside a cabinet mounted on a concrete pad.Although some electro-mechanical controllers are still in use ,
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.
Computerized traffic control box
In the United States, controllers are standardized by the NEMA, which sets standards for connectors, operating limits, and intervals.[1] The TS-1 standard was introduced in 1976 for the first generation of solid-state controllers.[3]
Traffic controllers use the concept of phases, which are directions of movement grouped together.[4] 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.[3] 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.[3]
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.
Pedestrian traffic signal in Taiwan, featuring a "Walking green man" below a countdown display where the "Red Man" once stood.
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 sensors 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 detectors. 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.
A Pegasus crossing in Hyde Park, London, England
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.[7] 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[8] buttons for users on horseback.[9][10]
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.
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 hours 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.
This traffic light in Khobar, Saudi Arabia
is video camera-actuated (just above the vertical light) and also shows
the seconds remaining to change to the next state (in the leftmost
horizontal light)
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[11]. Video cameras, or sensors 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 dollars
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:[12][13]
Traffic controllers use the concept of phases, which are directions of movement grouped together.[4] 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.[3] 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.[3]
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 timing of an intersection would benefit from being adapted to the dominant flows changing over the time of the day.[5]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 sensors 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 detectors. 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 EM 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 [6]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.[7] 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[8] buttons for users on horseback.[9][10]
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. Germany, 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 hours 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.
Benefits include:[12][13]
- 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'.
- New York City: 6,000 (of a total of 10,800) signalized intersections are controlled by a central computer[2]
- Toronto: 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.[14]
- Sydney: 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.[13][15][16]
- Melbourne: 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.[17]
- Adelaide: 580 sets of coordinated traffic lights throughout the metropolitan region managed by the Adelaide Coordinated Traffic Signal (ACTS) System.[12]
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.
- 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 priority
- Emergency Vehicles Some lights outside of fire or rescue stations have no green, as they may turn only amber and then red when fire trucks, ambulances, or other emergency vehicles of the like are exiting the station en route to an emergency. See also Traffic signal preemption
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