Review of Empirical Research on Congested Freeway Flow

In a review of the empirical literature on congested freeway flow, with special emphasis on topics of current interest, including congested flow phases, congested-regime flow-concentration relationships, wave speeds, average time-gap behavior, and transitions between uncongested and congested flow, the most striking feature of the results is their diversity. Shapes of flow-concentration relationships vary among studies and sites, wave speeds vary, patterns of repetition in congested flow appear to vary widely, and flow breakdown occurs at a variety of locations and for a variety of reasons. The diversity of results seems to indicate a similar diversity of phenomena rather than the effect of methodological differences. For the most part, methodologies are simple and intuitive, and because of their simplicity, they are unlikely to result in variability of the magnitude found in the results. Several methodological features that could possibly distort results are noted but do not appear to be responsible for the diversity of results. A major limitation of research to date is a lack of systematic understanding of the relative frequency of occurrence of different congested-flow phenomena. Because abundant loop-detector data are now available, research into the frequencies of congested-flow phenomena is feasible; however, analysis of very large quantities of data will be required.