Empirical Study of Traffic Features at a Freeway Lane Drop

Traffic was studied upstream and downstream of a bottleneck that arose near a freeway lane drop near London, U.K. using archived high-resolution loop detector data. The bottleneck’s location and mean discharge flows were reproducible from day to day. Further, it is shown that the bottleneck’s discharge flow was about 10% lower than the prevailing flow observed prior to queue formation. Upon bottleneck activation, flow reductions occurring sequentially in time and space marked the passage of the backward-moving shock. Mean shock velocities ranged between 4.8 and $6.4\mathrm{km}∕\mathrm{h}$ (3 and 4 mph) as they traveled upstream from the bottleneck. During bottleneck discharge, oscillations arose in the queue and propagated upstream at nearly constant speeds of $17.6\text{–}19.2\mathrm{km}∕\mathrm{h}$ (11–12 mph). Flows measured at locations downstream of the bottleneck were not affected by these oscillations. These findings were corroborated using data from a freeway lane drop in Minneapolis, Minn. The analysis tools used for this study were curves of cumulative vehicle count, time mean speed and occupancy versus time. These curves were constructed using data from neighboring freeway loop detectors and were transformed in order to provide the measurement resolution necessary to observe the transitions between freely flowing and queued conditions and to identify important traffic features.