Dynamics of social groups’ decision-making in evacuations

Abstract

The possible presence of social groups amongst occupants in indoor evacuation scenarios and the potential discrepancies between their behaviour and that of single individuals have been acknowledged in the evacuation dynamics literature. Yet, empirical studies on the behavioural characteristics of social-group evacuees, particularly those related to their decision-making mechanism, are relatively underrepresented. This study reports on a lab-in-the-field experiment of evacuations that investigates the effect of group size and stress level on multiple aspects of groups’ behaviour. These aspects include pre-movement time, decision time (i.e. time taken to reach a directional exit decision), exit-choice behaviour, exit-choice changing and intra-group decision dynamics. Our main findings showed that:(I)Group size had a significant effect on pre-movement times and decision times (and to lesser extent, on evacuation times). People took longer to initiate their movement or to make a directional exit decision when in groups compared to when acting alone. The influence of group size on pre-movement and decision times, however, was not monotonic. For smaller group sizes (sizes 2 and 3), pre-movement and decision times increased with the group size. But for the larger group size (size 4), the effect was reversed.(II)The exit-choice mechanism of groups, overall, showed a great degree of similarity to that of single individuals. This suggests that, from a modelling perspective, the directional exit choices of groups could be approximated with the models obtained from datasets of individually-made choices.(III)Groups were more likely to revisit and change their initial exit decisions under the higher level of stress. In addition, social influence was a major factor in triggering changes in the exit choices of groups. Nearly 50% of the changes in group decisions occurred shortly after another neighbour group changed its exit decision. In other words, instances of group decision-changing was often followed by another group taking a similar action.(IV)As to the within-group dynamics of decision-making, our analyses suggested that, regardless of the group size, leadership was the dominant group decision-making mechanism (compared to the consensus or the conform-to-majority mechanisms). In nearly 50% of the cases, group decisions were made by one individual taking the lead and hinting the direction of the movement to other members of the group.