Design of Traffic Signal
The conflicts arising from movements of traffic in different directions is solved by time sharing of the principle. The advantages of traffic signal includes an orderly movement of traffic, an increased capacity of the intersection and requires only simple geometric design. However the disadvantages of the signalized intersection are it affects larger stopped delays, and the design requires complex considerations. Although the overall delay may be lesser than a rotary for a high volume, a user is more concerned about the stopped delay.
Definitions and notations
A number of Definitions and notations need to be understood in signal design. They are discussed below:
Cycle: A signal cycle is one complete rotation through all of the indications provided.
Cycle length: Cycle length is the time in seconds that it takes a signal to complete one full cycle of
indications. It indicates the time interval between the starting of of green for one approach till the next
time the green starts. It is denoted by C.
Interval: Thus it indicates the change from one stage to another. There are two types of intervals -
change interval and clearance interval. Change interval is also called the yellow time indicates the interval between the green and red signal indications for an approach. Clearance interval is also called all red is included after each yellow interval indicating a period during which all signal faces show red and is used for clearing of the vehicles in the intersection.
Green interval: It is the green indication for a particular movement or set of movements and is denoted
by Gi. This is the actual duration the green light of a traffic signal is turned on.
Red interval: It is the red indication for a particular movement or set of movements and is denoted by
Ri. This is the actual duration the red light of a traffic signal is turned on.
Phase: A phase is the green interval plus the change and clearance intervals that follow it. Thus, during
green interval, non conflicting movements are assigned into each phase. It allows a set of movements to ow and safely halt the ow before the phase of another set of movements start.
Lost time: It indicates the time during which the intersection is not effectively utilized for any movement. For example, when the signal for an approach turns from red to green, the driver of the vehicle which is in the front of the queue, will take some time to perceive the signal (usually called as reaction time) and some time will be lost here before he moves.
Phase design
The signal design procedure involves six major steps. They include the (1) phase design, (2) determination of amber time and clearance time, (3) determination of cycle length, (4)apportioning of green time, (5) pedestrian crossing requirements, and (6) the performance evaluation of the above design. The objective of phase design is to separate the conflicting movements in an intersection into various phases, so that movements in a phase should have no conflicts. If all the movements are to be separated with no conflicts, then a large number of phases are required. In such a situation, the objective is to design phases with minimum conflicts or with less severe conflicts.
There is no precise methodology for the design of phases. This is often guided by the geometry of the intersection, ow pattern especially the turning movements, the relative magnitudes of ow. Therefore, a trial and error procedure is often adopted. However, phase design is very important because it affects the further design steps. Further, it is easier to change the cycle time and green time when ow pattern changes, where as a drastic change in the ow pattern may cause considerable confusion to the drivers. To illustrate various phase plan options, consider a four legged intersection with through traffic and right turns. Left turn is ignored. See figure 41:1. The first issue is to decide how many phases are required. It is possible to have two, three, four or even more number of phases.