  Human crowd motion is mainly driven by self-organized processes based on
local interactions among pedestrians. While most studies of crowd behavior
consider only interactions among isolated individuals, it turns out that up to
70% of people in a crowd are actually moving in groups, such as friends,
couples, or families walking together. These groups constitute medium-scale
aggregated structures and their impact on crowd dynamics is still largely
unknown. In this work, we analyze the motion of approximately 1500 pedestrian
groups under natural condition, and show that social interactions among group
members generate typical group walking patterns that influence crowd dynamics.
At low density, group members tend to walk side by side, forming a line
perpendicular to the walking direction. As the density increases, however, the
linear walking formation is bent forward, turning it into a V-like pattern.
These spatial patterns can be well described by a model based on social
communication between group members. We show that the V-like walking pattern
facilitates social interactions within the group, but reduces the flow because
of its "non-aerodynamic" shape. Therefore, when crowd density increases, the
group organization results from a trade-off between walking faster and
facilitating social exchange. These insights demonstrate that crowd dynamics is
not only determined by physical constraints induced by other pedestrians and
the environment, but also significantly by communicative, social interactions
among individuals.
