A figure-eight hysteresis pattern in macroscopic fundamental diagrams and its microscopic causes.

This paper presents macroscopic fundamental diagrams (MFDs) for an urban freeway network in Beijing, China. In the diagrams, a figure-eight hysteresis pattern is observed. To understand the causes, analyses are made ranging from spatiotemporal heterogeneity of vehicles to the flow-occupancy relation for individual locations. Eventually, it is observed at individual locations that free-flow traffic with the same occupancy exhibits different flows in the onset and offset of a rush hour; it is attributed to the counter-clockwise loop in the figure-eight hysteresis pattern at the macroscopic level. Through a simulation scenario based on a cellular automata (CA) model, different lane-changing rates are discussed as the microscopic causes of the flow-occupancy diagram with multiple free-flow branches. In practice, it is inevitable to collect lane-changing data sometimes because of the closeness between detectors and ramps. In the situation, the origin-destination (OD) matrices may influence the shape of MFDs. The paper enriches the knowledge about MFDs and also provides some empirical support for existing theories. [ABSTRACT FROM AUTHOR]