Saturation flow rate is the equivalent hourly flow rate at which previously queued vehicles can traverse an intersection approach under prevailing conditions, assuming that the green signal is available at all times and no lost times are experienced, in vehicles per hour of green or vehicles per hour of green per lane.

The graphic above illustrates how and when saturation flow rate typically occurs at a signalized intersection. As this graphic indicates, vehicles do not immediately begin passing through the intersection at saturation flow rate as soon as the traffic signal gives a green indication. Instead, there is a period of time when start-up delay or lost time is incurred as drivers react to the green indication and begin moving their vehicles. In the United States, saturation flow rate is assumed to be observable after the fourth vehicle in queue has crossed the stopline; in other countries, saturation flow rate is measured differently.

Note also that saturation flow rate does not necessarily sustain itself throughout the remainder of the green period. Saturation flow rate lasts only as long as it takes to pass the standing queue of vehicles through the intersection. Once those vehicles have passed through the intersection, then the flow rate will typically drop to something below the saturation flow rate, even if vehicles attach themselves to the end of the standing queue while the traffic signal light is green.