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 illuminated.

• The yellow or amber interval is the portion of the phase during which the yellow light is illuminated.

• 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 intervals.

• 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.