The normal function of traffic lights requires more than sight control and coordination to ensure that traffic and pedestrians move as smoothly and safely as possible. A variety of control systems are used to accomplish this, ranging from simple clockwork mechanisms to sophisticated computerized control and coordination systems that self-adjust to minimize delays for people using the junction.

In the United States, traffic signal timing is traditionally operated to minimize vehicle delay at traffic signals. This often affects the safety and mobility of people walking and riding bicycles.

History The first automated system for controlling traffic signals was developed by inventors Leonard Casciato and Josef Kates and was used in Toronto in 1954.Engelmann, Frederick C. (1996) A History of the Austrian Migration to Canada, Carleton University Press, , p. 184McLean, James W. (1966) "The Phony Ogre of Automation", Montreal Gazette, February 26, 1966, retrieved 2010-10-31"Josef Kates Found Ways to Unsnarl Traffic and Solve Business Problems With Computers". James R. Hagerty, Wall Street Journal, July 27, 2018

Terminology In Australia and New Zealand, the terminology is different. A "phase" is a period of time during which a set of traffic movements receives a green signal, equivalent to the concept of a "stage" in the UK. One electrical output from the traffic signal controller is called a "signal group" - similar to the UK's concept of "phase". PTV VISSIM also uses the signal group terminology.

In the United States, an "interval" is a portion of a signal cycle during which none of the signal indications change. A "phase" means the sum of right of way (green), change (yellow), and clearance (red) intervals assigned to an independent or combined traffic movement.

Phases

Phases are indications shown to traffic on traffic signal aspects (a single light on a signal head). For example, a green phase gives all traffic from a particular approach the right of way through the junction (bar turning traffic). In the UK, a filter phase allows non-conflicting traffic to make particular turns (normally left or ahead) through a junction.

A movement is any path through the junction which vehicles or pedestrians are permitted to take. A movement is conflicting if its paths cross. Normally, conflicting movements are not permitted, except for opposed right or left turns (depending on driving side) or, in some jurisdictions, pedestrians and vehicles moving in parallel directions.

Stages and periods A stage is a group of non-conflicting phases that move at the same time.Basics , Traffic Signal Training, NIATT / University of Idaho For example, a crossroads with four approach arms could operate in two-stage operation, where each road is given green, or three-stage operation, where the major road is given green, then each side road is given green in turn. A cycle is one complete sequence of stages.

The interstage or intergreen period is the period between the end of a green signal in one phase and the start of a green signal in the next phase. This normally includes an amber signal on approaches as the green phase ends, and an all-red stage, where all changing signals are red to allow the junction to clear. All red stages result in lost time, during which no road users can proceed through the junction.DfT Traffic Advisory Leaflet 1/06, Part 4 - https://tsrgd.co.uk/pdf/tal/2006/tal-1-06_4.pdf

An interval is the period between changes in signal stages. For example, the vehicular green interval is the period during which vehicular traffic has a green signal. The interval is fixed in pre-timed control and varied in actuated control. In actuated settings, the minimum interval is the shortest time a signal will remain green before changing. This can be as low as 2 seconds for local roads, but may need to be as long as 15 seconds for arterial roads. The maximum interval is the maximum amount of time one road will be allowed a green signal, where demand is present on another road.

For pedestrians, the invitation period is the time during which they are invited to begin crossing the road. This is normally shown with a green or white male walking figure.

Cycle time

The cycle time of an intersection, or set of coordinated signals, is typically the time it takes for all stages to complete, or the time from the start of the green light to the start of the next green. It tends to be longer at busier intersections and busier times of day.

Reducing cycle time can decrease delays for public transport vehicles and is particularly effective when they have their own lane or operate at high frequency.

Some jurisdictions have maximum cycle times. For example, in the UK, this is 120 seconds or 90 seconds where pedestrian facilities are present. Under actuated control, the reversion is the stage the traffic controller returns to when there is no demand. Maximum traffic signal cycle times are typical for Sydney Coordinated Adaptive Traffic System (SCATS) installations. The Sydney Coordinated Adaptive Traffic System (SCATS) includes nominal cycle times (nCL) and the 'as-run' (actual) cycle length (rCL).

Pedestrian safety

Shorter traffic signal cycle times enable shorter pedestrian wait times.

Shorter pedestrian wait times at signalised crossings reduce the likelihood of people walking on red (crossing the road against a signal), which improves safety.

Traffic controllers

A traffic signal is typically controlled by a controller mounted inside a cabinet.Traffic Signals 101 , Minnesota Department of Transportation, 2006 Some electromechanical 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 boxesChoreographing the Dance of Traffic Lights, New York Times, September 17, 1998). 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 turn off 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 Signal Standards , National Transportation Operations Coalition

Solid-state controllers must have an independent conflict monitor unit (CMU) to ensure fail-safe operation. The CMU monitors the controller's outputs, and if a fault is detected, it 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, for instance, green signals facing both northbound and eastbound traffic at a cross intersection. Conflict monitors are susceptible to false activation during thunderstorms due to power surges and noise induced by nearby lightning strikes.

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 attempted 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 stages, and they are usually coordinated so that stage changes occur in relation to other nearby signals, the press of a pedestrian button, the action of a timer, or other inputs.

Battery backup In areas prone to power interruptions, adding battery backups to traffic controller systems can enhance the safety of 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 a system would be prohibitive. With newer generations of traffic signals that use LED lights, which consume 85-90% less energy, it is now possible to incorporate battery backups into 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, the California Energy Commission recommended that local governments convert their traffic lights to LEDs with battery backups. This would reduce energy consumption and enhance safety at major intersections. The recommendation was for a system that provides fully functional traffic signals for two hours after the power outage. Then the signals will have flashing red lights for another two hours.

Types of control There are many types of control mechanisms for junctions controlled by traffic signals: +TypeMeaningConditionsExample useIsolated pre-timedFixed cycle lengthFor temporary operation, where detection is not availableRoadworksCoordinated pre-timedFixed cycle lengthWhere traffic is consistentCity centres, interchangesSemi-actuatedNo fixed cycle length, defaults to one movementTraffic imbalance - Highway operationsFully-actuatedNo fixed cycle length, detection used on all approaches, responsive to conditionsWhere detection is used on all roadsRural, high-speed locations or two arterial roadsCoordinated actuatedFixed cycle lengthHeavy traffic on arterial roadsSuburban arterial

Fixed time control

In traffic control, simple, older forms of signal controllers are known as electromechanical signal controllers. Unlike computerized signal controllers, electromechanical signal controllers are primarily composed of movable parts (cams, dials, and shafts) that control signals wired directly to them. In addition to movable parts, electrical relays are also used. In general, electromechanical signal controllers use dial timers with fixed, signalized intersection timing 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 fails, 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 electromechanical signal controllers, and signals controlled by them are effective in one-way grids, where it is often possible to coordinate them with 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.Learning Traffic Light Control Policies URL accessed 2009-03-06

Coordinated control