The popular concept is that coordinating traffic signals is simply to provide green-wave progression whereby a motorist travelling along a road receives successive green signals. While this is one of the aims, the principal purpose of the control system is to minimise overall stops and delay and, when traffic demand is at or near the capacity of the system, to maximise that capacity (throughput) and minimise the possibility of traffic jams by controlling the formation of queues.
Most area traffic control systems control the signals on a fixed-time basis in which a series of signal timing plans are brought into operation at certain times of the day. Each plan determines the timing of individual signals and the time relationship between signals and is precalculated, based on previously surveyed traffic conditions. As such, a fixed-time system is unable to cope with unpredictable traffic conditions. Furthermore, as traffic conditions change with the passage of time, fixed time plans become inappropriate. This requires the area to be re-surveyed and new signal timing plans calculated every few years. Experience has shown this procedure to be expensive and to require resources which are not always readily available. As a result, the development of new plans is either deferred beyond the useful life of the old plans or ad hoc changes are made to the plans and timetables, usually resulting in sub-optimum performance.
The problems of fixed-time systems suggest that a technique more responsive to changing traffic conditions (rather than the time of the day) would be more appropriate and more acceptable to the motoring public who, in Australia, have for many years been accustomed to the responsiveness of fully vehicle actuated isolated intersection control.
A fully responsive system, unlike fixed-time, semi-responsive or pattern matching systems (often claimed to be fully responsive or adaptive), requires no precalculation of signal timing plans. Logic and algorithms in a truly on-line traffic control computer analyse real-time traffic data from vehicle detectors to produce signal timings which are suitable for the average prevailing traffic conditions.
The implementation of a fully responsive system does not, however, mean that the careful design of each intersection can be avoided. The present state of technology only allows for the real-time variation of signal timings at the intersections which have been designed to suit known or anticipated traffic requirements. A degree of adaption of the local design to varying traffic requirements can be accommodated by a system such as SCATS. This is done by providing a variable sequence of phases and the ability to omit phases or movements from the sequence on a cycle by cycle basis, when there is no demand.