Empirical evaluation of influential factors on bifurcation in macroscopic fundamental diagrams

Abstract

Observations from empirical data in the roadway network showed that the relation between averages of network flow versus density often exhibit hysteresis and bifurcation phenomena, which may obscure the reproducibility of a well-defined macroscopic fundamental diagram (MFD). In this paper, we analyzed large-scale trip data from passenger vehicles in an urban network of South Korea and evaluated the shapes of MFDs over many days. It was found that MFDs have two distinctive, reproducible forms: a well-defined, unique relation on weekends and a bifurcation in high-density regime on weekdays. With regard to the bifurcation, we observed higher network flows in the morning and lower network flows in the evening for the same average network density. This implies that the same set of drivers in the network collectively formed two different trip patterns. Hence, we evaluated possible factors that may have effects on the bifurcation phenomenon. In view of this, four factors - heterogeneity, trip completion rate, detouring ratio and commute trips - were analyzed in this study. The findings showed that travelers' detours could be a key factor for the occurrence of bifurcation because, by detouring travelers improve neither their own travel times nor network wide travel times, and thereby degrade the network production.