  Being determined by human social behaviour, pedestrian group dynamics depends
on "intrinsic properties" of the group such as the purpose of the pedestrians,
their personal relation, their gender, age, and body size. In this work we
quantitatively study the dynamical properties of pedestrian dyads by analysing
a large data set of automatically tracked pedestrian trajectories in an
unconstrained "ecological" setting (a shopping mall), whose relational group
properties have been coded by three different human observers. We observed that
females walk slower and closer than males, that workers walk faster, at a
larger distance and more abreast than leisure oriented people, and that inter
group relation has a strong effect on group structure, with couples walking
very close and abreast, colleagues walking at a larger distance, and friends
walking more abreast than family members. Pedestrian height (obtained
automatically through our tracking system) influences velocity and abreast
distance, both growing functions of the average group height. Results regarding
pedestrian age show as expected that elderly people walk slowly, while active
age adults walk at the maximum velocity. Groups with children have a strong
tendency to walk in a non abreast formation, with a large distance (despite a
low abreast distance). A cross-analysis of the interplay between these
intrinsic features, taking in account also the effect of extrinsic crowd
density, confirms these major effects but reveals also a richer structure. An
interesting and unexpected result, for example, is that the velocity of groups
with children {\it increases} with density, at least in the low-medium density
range found under normal conditions in shopping malls. Children also appear to
behave differently according to the gender of the parent.
