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.
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.
Read more about this topic: Traffic Light Control And Coordination
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