The following excerpts were taken from the 1994 Highway Capacity Manual, published by the Transportation Research Board.
Capacity at signalized intersections is based upon the concept of saturation flow and saturation flow rate. Saturation flow rate is given the symbol s and is expressed in units of vehicles per hour of effective green time (vphg) for a given lane group.
The flow ratio for a given lane group is defined as the ratio of the actual or projected demand flow rate for the lane group (vi) to the saturation flow rate (si). The flow ratio is given the symbol (v/s)i (for lane group i).
The capacity of a given lane group may be stated as
Saturation flow rate is defined as the flow rate per lane at which vehicles can pass through a signalized intersection in such a stable moving queue. By definition, it is computed as
s = 3,600/h