A City-Scale Macroscopic Fundamental Diagram for Mixed Bi-modal Urban Traffic

Recent research has studied the existence and the properties of macroscopic fundamental diagrams (MFDs) for urban areas. These MFDs should not be universally expected as high scatter or hysteresis might appear for some type of networks, like heterogeneous networks or freeways. In this paper, we investigate if aggregated relationships can describe the performance of urban bi-modal networks with buses and cars shared the same road infrastructure. Based on simulated data, we develop a three-dimensional MFD (3D-MFD) relating the accumulation of cars and buses, and the total circulating flow in the network. This relation experiences low scatter and can be approximated by an exponential-family function. Given these observations, we propose an elegant model to estimate a passenger 3D-MFD which provides a different perspective of the flow characteristics in bi-modal networks. We also show that a constant Bus-Car Unit (BCU) equivalent value cannot describe the effect of buses in system congestion. We then integrate a partitioning algorithm to cluster the city into a small number of regions with similar mode composition. Our results show that the partitioning unveils important traffic properties of flow heterogeneity in the studied network. Various traffic management strategies in bi-modal urban road networks can be integrated, such as redistribution of urban space among different modes, perimeter signal control with preferential treatment of buses and bus priority.