Characterizing crowd state dynamics in confined space during simulated violent attack: Insights from controlled experiments

The mechanism by which sudden mobile threats impact crowd evacuation in confined spaces is a significant research topic in the field of complex dynamics. Focusing on high-density confined spaces such as subway security checkpoints, this study conducted controlled evacuation experiments simulating violent attacks. The experiments involved 120 participants divided into attackers and evacuees, covering 30 scenarios with varying threat intensities and dynamic boundary conditions. A moving polar coordinate system centered on the attacker was established, and the evolutionary laws of crowd states were quantitatively analyzed based on high-precision microscopic trajectory data. The results show that the attack threat exhibits strong anisotropy, forcing the surrounding crowd into an asymmetric dispersion, while incapacitated individuals transformed into stationary physical obstacles directly induce local “congestion nucleation”. Macroscopically, this perturbation triggers a behavioral phase transition in the crowd system. At the critical density interval of 3.0 to 4.0 ped / m 2 , the system undergoes a forced shift from a “threat-dominated phase” to a “density-dominated phase”. High-intensity threats rapidly deplete the pool of actively escaping individuals, leading to a premature truncation of the evacuation process and a step-like collapse in the escape rate. Furthermore, external threats acting as dynamic boundary conditions sever optimal escape routes and trigger symmetry breaking in the spatial flow network. The inverse collision of outward-escaping and retreating human flows forms a fatal “bidirectional dynamic compression”, resulting in a significant increase in the casualty rate. This study reveals the mechanisms of spatial reorganization and behavioral phase transition in crowd flow fields under extreme threats within confined spaces, providing a scientific basis for formulating emergency response plans in public venues.