Estimation of the Network Fundamental Diagram (NFD): An Urban Application in Emergency Conditions

The control of urban road networks by means of traffic signals has been an area of intensive research for decades. Several models, algorithms and tools have been developed and implemented to support the management and design of urban networks. Nevertheless, researchers are still working in this field proposing new methods to improve mobility on urban networks in saturated (ordinary and emergency) traffic conditions. According to the recent literature, the global traffic conditions of an urban road network in ordinary conditions may be measured with good approximation by an asymmetric inverse-U shaped diagram called Network Fundamental Diagram (NFD). More specifically, the NFD puts in relation the number of vehicles present in the network with the (link-length weighted) total network traffic flow, in accordance with the fundamental diagram used for motorway links.As a case study, the methodology is applied to the urban network of Melito Porto Salvo (Italy), where a forthcoming disaster is simulated during the morning rush hour of a working day in accordance with the emergency plan drawn up by the Civil Protection Department. The evacuation phases of population from the area with motorized modes, are simulated by means of a dynamic assignment model, previously calibrated and validated. The NFD is compared with the objective function of an optimized network design approach.The objective of the paper is to verify if a global indicator, as it is the Network Fundamental Diagram, could be able to reflect the evolution of traffic conditions and to estimate the effects of control decisions of urban road networks (e.g. by means of traffic signals) during an evacuation phase, as the one described in the application.