Non-unimodal and non-concave relationships in the network Macroscopic Fundamental Diagram caused by hierarchical streets

Unimodal, concave relationships between average network productivity and accumulation or density aggregated across spatially compact regions of urban networks—so called Macroscopic Fundamental Diagrams (MFDs)—have recently been shown to exist on homogeneous street networks. When present, MFD facilitate the modeling traffic congestion at a regional level have led development various control strategies. However, real networks are not homogeneous—they generally hierarchical structure where some streets (e.g., arterials) promote higher mobility than others local roads). This paper examines how presence roadway structures may potentially cause non-unimodal patterns in network's MFD. These observed using three types tools: analytical models simple structures, simulations idealized networks, empirical data. The impacts hierarchy depend vehicles use different move within network; i.e., their routing strategy. findings suggest that hierarchies lead MFDs non-concave free-flow branch when route themselves according user equilibrium principles, which is closest what would be realistic situations. Such contrary traditionally assumed most MFD-based frameworks. unimodal should expected under system optimal conditions maximize for given state.