In collaboration with Fadratul H. HASSAN (Malaysia), we have just published a (hopefully) pedagogical review on the current knowledge about the impact of social groups on crowd dynamics.
How is your motion altered when you are moving along with other people, rather than alone? Does this impact the collective flow properties of the crowd?
Surprisingly, these questions were left aside for a long time after the development of the first agent-based models for pedestrian dynamics. When they were eventually carefully studied, researchers noticed remarkable patterns in the 'microscopic' organisation of social groups in crowds. In particular, groups of three people tend to walk in a characteristic V-shape, with the central pedestrian slightly backwards with respect to their neighbours -- which is exactly the opposite of the pattern observed for bird flocks! While these microscopic features are now well established, their effect on the collective crowd dynamics remain more controversial, as we relate in our review.