  Understanding the collective dynamics of crowd movements during stressful
emergency situations is central to reducing the risk of deadly crowd disasters.
Yet, their systematic experimental study remains a challenging open problem due
to ethical and methodological constraints. In this paper, we demonstrate the
viability of shared 3D virtual environments as an experimental platform for
conducting crowd experiments with real people. In particular, we show that
crowds of real human subjects moving and interacting in an immersive 3D virtual
environment exhibit typical patterns of real crowds as observed in real-life
crowded situations. These include the manifestation of social conventions and
the emergence of self-organized patterns during egress scenarios. High-stress
evacuation experiments conducted in this virtual environment reveal movements
characterized by mass herding and dangerous overcrowding as they occur in crowd
disasters. We describe the behavioral mechanisms at play under such extreme
conditions and identify critical zones where overcrowding may occur.
Furthermore, we show that herding spontaneously emerges from a density effect
without the need to assume an increase of the individual tendency to imitate
peers. Our experiments reveal the promise of immersive virtual environments as
an ethical, cost-efficient, yet accurate platform for exploring crowd behaviour
in high-risk situations with real human subjects.
