An intersection is a location where two or more roads carrying traffic streams in different directions cross. The space which is common to all these roads is referred to as the intersection. At such a location, obviously, different traffic streams compete with one another for the use of the common space or the intersection. If left on its own the flow at an intersection will always be chaotic; the safety and efficiency at such locations will be low. Hence, various strategies are used to control the flow of traffic at an intersection in order to improve the safety and efficiency of traffic flow. Among the strategies that are used, signalization is common.
At a signalized intersection, the common space is periodically given to certain flows while other conflicting streams are barred from entry at that time. In a manner of speaking the common space is time-shared among the various flows. Although, there are various kinds of time sharing strategies like pre-timed, partially actuated, and fully actuated signalizations, in this section the pre-timed signalization and its effect on flow is studied. The other strategies, where the time sharing mechanism changes more frequently than in the pre-timed strategy are not studied here as the basic traffic flow analysis process is the same as that of the pre-timed strategy.
In the pre-timed signalization, the time sharing between the different conflicting flows occurs according to a pre-defined strategy which repeats at a fixed interval. This fixed interval is referred to as the cycle length. During the cycle length, the time for which a particular stream can utilize the intersection is referred to as the green time for that stream or movement, the time during which a particular movement cannot utilize the intersection is referred to as the red time for that movement. Invariably during the change over from green to red an amber signal is shown to warn the driver that a red signal is impending. During the amber time for a movement the vehicles of that movement can use the intersection. Of course the sum of green, amber and red times for a particular movement is equal to the cycle time. A detailed discussion and design of pre-timed signalization is provided in the next chapter. The above introduction is given briefly here in order to initiate the reader to the various terminology used in signalized intersection analysis and design.
Obviously, at a signalized intersection, there is interruption to flow of traffic. The type of interruption and its effect on the flow is described in the next several sections.