Evacuation performance of individuals and social groups under different visibility conditions : Experiments and surveys

The sooty smoke generated from fire reduces visibility at fire scenes. This not only slows down evacuation, but also affects the mental states of evacuees. A series of evacuation experiments were conducted in a room under different visibility conditions. The crowd was formed by mixing individuals and social groups. Critical results regarding the evacuees’ travel time, pre-movement time, movement speed and typical behaviour, such as leadership behaviour and following behaviour, were collected. It was found that evacuation time increased, movement speed decreased and flow at exits became discontinuous when visibility was reduced. Group behaviour had a negative effect on evacuation in normal visibility, but the group effect was basically positive in the reduced visibility scenarios. A number of interesting findings were generated. Larger sized groups tend to move side by side, forming a horizontal structure. Smaller sized groups are more likely to form a vertical structure. Similarly, groups with a leader demonstrated vertical movement, distinct from the horizontal movement of groups without a leader. Following behaviour was more notably observed and is advantageous to overall evacuation when visibility reduced. The results of the pre-experiment questionnaire showed that over 80% of the participants walk in groups on campus. The daily walking groups tended to maintain their moving forms even in the case of an emergency evacuation. Physical touch was more widely demonstrated in the emergency groups than in the daily groups. This study furthers the understanding of human behaviour in different visibility evacuation conditions and recognition of the impacts of social groups on crowd dynamics.