Dynamics of Urban Network Traffic flow during a Large-Scale Evacuation

This paper explores some of the dynamics of urban network traffic flow during a large-scale evacuation in the context of the network fundamental diagram (NFD). The structure of the evacuation demand can significantly affect network performance. A radial-shaped structure results in a more stable network recovery compared with a directional evacuation structure. This study confirms the existence of unloading–reloading hysteresis when a network is subject to successive cycles of loading and unloading. If a network undergoes a complete or near-complete recovery, the reloading path in the NFD follows almost the same path as in the initial loading. Results suggest that the linear relationship between average network flow and trip completion rate does not always hold, as previously thought. The relationship becomes highly scattered and nonlinear when the network is highly congested and under disruption and the number of adaptive drivers is sufficiently large. Frequent route switching by adaptive drivers can artificially increase the average network flow but does not necessarily increase the network output (trip completion rate). Adaptive driving increases fluctuations in the NFD; however, it reduces hysteresis and gridlock while increasing network capacity.