Analysis of the Breakdown Process on Congested Freeways

The aim of the study is to model the flow breakdown phenomenon on congested freeways. Because of the importance of the breakdown phenomenon, the present research focuses on analyzing, understanding, and modeling heavy-flow breakdowns when no apparent bottlenecks exist. It is hypothesized that breakdowns are caused by internal microscopic flow oscillations and the transition from dense flow to unstable flow on freeways. A new density-versus-time model was developed to describe this phenomenon. The model shows the density behavior over time, which increases sharply toward the peak period and then decreases and increases again toward the breakdown. This new revelation is described by a combination of an exponential model, a logistic model, and a weighting function. This density–time relationship is calibrated by using data collected from a congested freeway at 1-min intervals. The beginning of the unstable flow is defined as the drop in speed below 60 km/h; it is shown that breakdown is defined as a process that starts at the local minimum of the new model and ends at the beginning of the unstable flow. It was found that the model well describes the behavior of density over time, from the increase in density through the breakdown process and into the rapid density-fluctuation phase of the unstable flow.