In order to understand the operation of traffic signals it is first necessary to understand the basic concepts of movements and phases.
Each possible trajectory of traffic flow is called a movement. At a typical four-way intersection, each approach to the intersection can accommodate three movements:
- vehicles turning left
- vehicles travelling straight through
- vehicles turning right.
In the simple intersection shown in Figure 6.5 there are three movements in each approach for a total of 12 movements.
The phase is the entity which the controller uses to share time among the various compatible movements. A phase may consist of a set of non-conflicting movements or certain conflicting movements where the priority is defined by traffic regulations. Where a phase contains conflicting movements, those movements which are obliged to give way are said to be filter movements.
The maximum of seven phases operated by a controller are labelled A to G. In some cases, phases may have options within the same phase, e.g. E, E1 and E2. Only one phase can be ‘running’ at any one time. Phases are typically serviced in alphabetical order (although this is not essential) and phases may be skipped if they are not demanded. Selecting a phasing design for a particular intersection depends on the traffic flows of vehicles and pedestrians for each movement.