While each phase of a cycle can service several movements or lanes, some of these lanes will inevitably require more time than others to discharge their queue. For example, the right-turn movement of an approach may service two cars while the straight-through movement is required to service 30 cars. The net effect is that the right-turn movement will be finished long before the straight-through movement. What might seem to be an added complexity is really an opening for simplicity. If each phase is long enough to discharge the vehicles in the most demanding lane or movement, then all of the vehicles in the movements or lanes with lower time requirements will be discharged as well. This allows the engineer to focus on one movement per phase instead of all the movements in each phase.
The movement or lane for a given phase that requires the most green time is known as the critical movement or critical lane. The critical movement or lane for each phase can be determined using flow ratios. The flow ratio is the design (or actual) flow rate divided by the saturation flow rate. The movement or lane with the highest flow ratio is the critical movement or critical lane. You will see how this concept is applied in the cycle length and green split discussions.