The normal function of traffic lights requires more than slight 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.
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Phases and stages
Traffic controllers use the concept of phases, which are directions of movement grouped together. For instance, a simple crossroads may have four vehicle movement phases: North, East, West and South. There may be additional phases for pedestrian movements as well.
A stage is a group of phases which run at the same time. A simple crossroads may have two stages: North and South, and West and East. It is important that phases in a stage do not conflict with each other.
Traffic controller systems
A traffic signal is typically controlled by a controller inside a cabinet mounted on a concrete pad. Some electro-mechanical controllers are still in use (New York City still had 4,800 as of 1998, though the number is lower now due to the prevalence of the signal controller boxes). However, 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.
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 stage and they are usually coordinated so that the stage 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.
Battery backup
In the areas that are prone to power interruptions, adding battery backups to the traffic controller systems can enhance the safety of the motorists and pedestrians. In the past, a larger capacity of uninterruptible power supply would be required to continue the full operations of the traffic signals using incandescent lights. The cost for such system would be prohibitive. After the newer generations of traffic signals that use LED lights which consume 85-90% less energy, it is now possible to incorporate battery backups into the traffic light systems. The battery backups would be installed in the traffic controller cabinet or in their own cabinet adjacent to the controller.
The battery backups can operate the controller in emergency mode with the red light flashing or in fully functional mode. In 2004, California Energy Commission recommended to have local governments to convert their traffic lights to LEDs with battery backups. This would lower the energy consumption and enhance the safety at major intersections. The recommendation was for a system which provides fully functional traffic signals for two hours after the power outage. Then the signals will have flashing red lights for another two hours.
Fixed time control
In traffic control, simple and old forms of signal controllers are what are known as electro-mechanical signal controllers. Unlike computerized signal controllers, electro-mechanical signal controllers are mainly composed of movable parts (cams, dials, and shafts) that control signals that are wired to them correctly. Aside from movable parts, electrical relays are also used. In general, electro-mechanical signal controllers use dial timers that have fixed, signalized intersection time plans. Cycle lengths of signalized intersections are determined by small gears that are located within dial timers. Cycle gears, as they are commonly known, range from 35 seconds to 120 seconds. If a cycle gear in a dial timer results in a failure, it can be replaced with another cycle gear that would be appropriate to use. Since a dial timer has only one signalized intersection time plan, it can control phases at a signalized intersection in only one way. Many old signalized intersections still use electro-mechanical signal controllers, and signals that are controlled by them are effective in one way grids where it is often possible to coordinate the signals 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.
Dynamic control
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 controller's 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 and bicycles or vehicles 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 electromagnetic waves, or acoustic sensors to detect the presence of vehicles at the intersection waiting for the 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. They also act as multi-lane detectors and collect data types not available from in-roadway sensors.
Non-motorized user detection
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; this turns off when the vehicular lights enter the "red" phase. In the United States, the pedestrian signals continue to display a steady red "hand" or "Don't Walk" signal when the button is pressed, turning to a white "man" or "Walk" signal at the end of the vehicular 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 ferrous metal content of typical bicycles. 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 a green wave, a long string of green lights (the technical term is progression). 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 (progressed) systems are controlled from a master controller and are set up so lights "cascade" (progress) 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 from the 1940s, but are less common today. In the UK, Slough in Berkshire had part of the A4 experimented on with this. Many US cities set the green wave on two-way streets to operate in the direction more heavily traveled, rather than trying to progress traffic in both directions. But the recent introduction of the flashing yellow arrow (see Traffic-light signalling and operation) makes the lead-lag signal, an aid to progression, available with protected/permissive turns.
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 get through 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 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:
Examples: