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Stability of Freeway Bottleneck Flow Phenomena

Data for 44 days from five extended freeway sections around bottlenecks in the San Diego, California, area were analyzed to determine the stability of the point of initial flow breakdown and the feasibility of using similar data for more extensive research into the stability of bottleneck flow phenomena. The ultimate goal of such research is to shed light on the nature of transitions from uncongested to congested flow. Analysis of speed drop sequences suggests that there is rarely a single bottleneck location within critical freeway sections. This in turn suggests that many bottlenecks should be thought of as extended sections rather than points or isolated segments. This suggests an understanding of flow transitions intermediate between the conventional view that flow breaks down consistently at a few locations and the view that flow breakdown is spontaneous and that congested flow is self-organized. Data similar to those used in this study are adequate, but not ideal, for further investigation of the stability of bottleneck flow phenomena. Specific limitations relate to the locations of detector stations and the presence of chronic data errors. This approach to the study of bottlenecks can be improved by combining direct observation with analysis of loop detector data and by using cumulative flow counts to estimate changes in the numbers of vehicles stored in freeway segments.