Investigation of the early-onset breakdown phenomenon at urban expressway bottlenecks in Shanghai

Based on the analyses of recurring bottlenecks in Shanghai's expressways using spatial–temporal diagrams, the method of transformed cumulative curves was used to investigate the traffic flow characteristics revealed by loop detector data at two typical isolated bottlenecks (a lane-drop and an on-ramp). The pre-queue flow (PQF) and queue discharge flow (QDF) of the two bottlenecks were analysed. The QDFs were an average of 18% and 27% higher than the PQF in the lane-drop and on-ramp bottlenecks, respectively. These findings differ from the results of bottlenecks in other countries (e.g. the M4 motorway near London, UK; I-494 in Minneapolis, MN, USA; the Queen Elizabeth Way (QEW) and the Gardiner Expressway in metropolitan Toronto, Canada) and indicate that lane-drop and on-ramp bottlenecks in Shanghai experience early onsets of traffic breakdown. The potential causes of the early onset of breakdown were discussed from the perspectives of driving behaviour, the temporal features of the flow rate and the features of merging.

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