Investigation of Early-Onset Breakdown Phenomenon at Urban Expressway Bottlenecks in Shanghai, China

Based on the analyses of recurring bottlenecks in Shanghai’s expressways using spatial-temporal diagram, three typical isolated bottlenecks (a lane drop, an on-ramp, and a weaving section) were selected and method of transformed curves was adopted to investigate the traffic flow characteristic using the loop detector data. The starting and ending times of the three kinds of bottlenecks, pre-queue flow (PQF), queue discharge flow (QDF) were analyzed. Finally, some conclusions were discovered that QDF was higher than PQF in bottleneck sections of lane drop and on-ramp, the average difference were 18% and 27% respectively, but the situation was different in weaving section, the average rate of reduction was 22%. The findings were obviously different from results of diverse bottlenecks in other countries (e.g. M4 motorway near London, United Kingdom, I-494 in Minneapolis, Minnesota, USA, the Queen Elizabeth Way (QEW) and the Gardiner Expressway in metropolitan Toronto, Canada), i.e., the bottlenecks in lane drop section and on-ramp bottleneck have the early-onset characteristics. At last, the reasons of early-onset breakdown were preliminarily discussed from the aspects of driving behaviors, the temporal feature of flow rate and the feature of merging.