Tracing congestion

Upon arriving at the tail of a queue, drivers often wonder about the cause for the congestion they just drove in. Other drivers are surprised to see that at a merge, congestion often does not start in the merging area itself, but a substantial distance downstream of the merge (the so-called capacity funnel). Observations show the ill-predictability of the emergence of congestion, in terms of where and when the queues start building up and disappear. Finally, the issue of the so-called capacity drop is still not dealt with adequately, lacking a generic theory of the causes for the differences in pre-queue capacity and queue discharge rate.

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Science has not yet been successful in explaining these puzzling but interesting phenomena of congested traffic flows. The reasons that these phenomena are poorly understood are two-fold: on the one hand, a satisfactory generic and valid theory of traffic flows, in particular of car-following and lane-switching behavior of individual drivers during congestion is not available. On the other hand, adequate data needed to develop this theory are lacking. Common measurement methods collect only 'snapshots' of the situation (i.e. at a limited number of cross-sections, or a single, instrumented vehicle). Such data do not provide sufficient information to study the dynamics of individual drivers in their continuously changing traffic and roadway environment.

As a result, current flow models are in general not able to predict correctly traffic flow during congestion, especially from the viewpoint of the individual driver.