Saturation flow is the basis for the determination of traffic signal timings and for the evaluation of intersection performance. Most current analysis and design methods are based on some form of the critical lane technique, in which the critical lane, or group of movements, is determined from the relationship between the volume it carries and its saturation flow. The general descriptions of saturation flow presented in the highway capacity manual, the canadian capacity guide for signalized intersections, and an australian road research board special report on traffic signal capacity and timing analysis are more or less identical for similar close-to-ideal intersection conditions. Nevertheless, the definitions implied by the recommended survey techniques are significantly different. Moreover, saturation flow values reported by some researchers in the past have been based on measurement methods that differ from all three documents. Thedifferences in the meaning of saturation flow, as understood in different regions and by different authors, are discussed. The implications are illustrated using the three documents and a set of 1989 edmonton surveys as examples. It is found that values reported in the literature may not be directly comparable and that saturation flows measured with the assistance of one document may not be compatible with calculation techniques taken from another. This paper appears in transportation research record no. 1320, Freeway operations, highwaycapacity, and traffic flow 1991.
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