Time-dependent traffic flow features at a freeway bottleneck downstream of a merge

This dissertation describes an empirical study of the spatial and temporal evolution of traffic conditions upstream and downstream of a freeway bottleneck located near a busy on-ramp. The diagnostic tools used in this study were curves of cumulative vehicle arrival number versus time and cumulative occupancy versus time constructed from data measured by neighboring freeway loop detectors. Once suitably transformed, these cumulative curves provided the measurement resolution necessary to observe the transitions from freely-flowing to queued conditions and to identify some notable, time-dependent traffic features in and around the bottleneck.; On the eight days analyzed, it is shown that the bottleneck always occurred at a fixed location, approximately one kilometer downstream of the merge. Most notably, no matter what happened prior to bottleneck activation, the discharge flow in the active bottleneck was nearly constant, since the cumulative counts never deviated much from a linear trend. The average bottleneck discharge flow was reproducible from day to day in each lane and in total. Therefore, it appears that this long-run queue discharge flow should be viewed as the bottleneck capacity given that the near-constant rates were sustained for prolonged periods and that they were replicated (approximately) each day.; On the four days when the bottleneck became active without interference from any downstream effects, it is shown that flow dropped substantially following the formation of an upstream queue. On such days, reproducible signals (or cues) of impending queue formation were observed for the final few minutes prior to queue formation, including the achievement of a very high flow of approximately 7,000 vph across all three freeway travel lanes. On these days, it appeared that the higher flows observed prior to queue formation were unstable, as they varied in their magnitudes and in the durations that they persisted each day.; It was also found that on four other days, the subject bottleneck was activated only after a queue emanating from somewhere further downstream had dissipated. On these days, vehicles immediately began discharging through the bottleneck at a nearly constant rate without ever exhibiting a higher flow.; Given that little is known about how traffic flows through bottlenecks, a greater understanding is required to formulate, to enhance or to verify mathematical models of vehicular traffic, so that they are consistent with the actual traffic features that are found to be reproducible. This understanding is also required before one can conclude whether or not bottleneck flows can be increased by eliminating or postponing freeway queues with control measures such as ramp metering.