FALLACIES AND IMPLICATIONS OF CONVENTIONAL SATURATION FLOW MODEL OF QUEUE DISCHARGE BEHAVIOR AT SIGNALIZED INTERSECTIONS

Current methodologies used in many countries for capacity and level-of-service analysis of signalized intersections are based on the concept of saturation flow. They assume that, after green onset, the discharge rate of queuing vehicles will quickly reach its saturation flow rate after four or five vehicles have entered the intersection, and that the saturation flow will be sustained until shortly after the signal change interval begins. Recent studies, however, show that this saturation flow model often cannot realistically represent the observed queue discharge behaviors. Therefore continued use of the traditional model will have serious implications. This paper uses field data collected in Taiwan and the United States to analyze first the discrepancies between the observed queue discharge behaviors and the saturation flow model. It then discusses the implications of continued use of saturation flow for capacity estimation.