Investigation of Some Characteristics of Congested Flow

Recent German research has suggested that there may be two qualitatively different types of congested flow, referred to as “traffic jams” and “synchronized flow.” The possible existence of synchronized flow raises a theoretical issue, since it is reported to be characterized by randomly sloped flow-density “transferences” (lines connecting adjacent points in the time series), and this may imply that there is no consistent wave direction in such flow. An analysis of flow-occupancy data from San Diego suggests that on the basis of the appearance of the data alone, two types of congested flow may be identified. These appear to be similar to, but not identical with, the two types reported in the German work; in particular, a region with positively sloped flow transferences was found. Further consideration of the data and the mechanics of congested flow suggests, however, that these two types of flow do not represent qualitatively different types of traffic behavior but rather are the result of random variations in average time gaps in the traffic stream and the relationship among speed, flow, density, and average time gap. In particular, predominantly positively sloped flow-concentration transferences do not necessarily imply that waves move downstream; rather they imply that, for data taken over short time intervals (30 s or 1 min), transferences are not a good indicator of kinematic wave speed.