Signalized intersections permit conflicting traffic movements to proceed
efficiently and safely through space that is common to those movements. This is
accomplished by separating the individual movements in time rather than in space.
The various movements are collected and allowed to move in turn, or in phases. Each
phase of a signal cycle is devoted to only one collection of movements. These movements
are those that can proceed concurrently without any major conflict. For example, the
straight-through and right-turn movements of a street can be permitted to use an
intersection simultaneously without any danger to the motorists involved. This might be
one phase of a multi-phase cycle.
Some movements are allowed to proceed during a phase even though they cause conflicts.
Pedestrians are commonly allowed to proceed across intersections even though right-turn
movements are occurring. These movements are called permitted, while protected movements
are those without any conflicts.
In any case, the movements at an intersection can be grouped, and then these groups can
be served during separate phases.
The basic timing elements within each phase include the green interval, the effective
green time, the yellow or amber interval, the all-red interval, the intergreen interval,
the pedestrian WALK interval, and the pedestrian crossing interval. Each of these elements
is described below.
- The green interval is the period of the phase during which the green signal is
- The yellow or amber interval is the portion of the phase during which the yellow light
- The effective green time is contained within the green interval and the amber interval.
The effective green time, for a phase, is the time during which vehicles are actually
discharging through the intersection.
- The all-red interval is the period following the yellow interval in which all of the
intersection's signals are red.
- The intergreen interval is simply the interval between the end of green for one phase
and the beginning of green for another phase. It is the sum of the yellow and all-red
- The pedestrian WALK interval is the portion of time during which the pedestrian signal
says WALK. This period usually lasts around 4-7 seconds and is completely encompassed
within the green interval for vehicular traffic. Some pedestrian movements in large cities
are separate phases unto themselves.
- Finally, the pedestrian crossing time is the time required for a pedestrian to cross the
intersection. This is used to calculate the intergreen interval and the minimum green time
for each phase.
- This brief look at the basic signal timing elements should help you navigate through the
rest of the signal timing design concepts. Please remember to visit the glossary if you
aren't sure about a definition.