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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
 .
- 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 and is provided after each
yellow interval indicating a period during which all signal faces show red and
is used for clearing off the vehicles in the intersection.
- Green interval: It is the green indication for a particular
movement or set of movements and is denoted by
 .
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
 .
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 flow and safely halt the flow 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 before vehicle actually moves and gains speed.
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