  This article is on collective phenomena in pedestrian dynamics during the
assembling and dispersal phases of gatherings. To date pedestrian dynamics have
been primarily studied in the natural and engineering sciences. Pedestrians are
analyzed and modeled as driven particles revealing self-organizing phenomena
and complex transport characteristics. However, pedestrians in crowds also
behave as living beings according to stimulus-response mechanisms or act as
human subjects on the basis of social norms, social identities or strategies.
To show where pedestrian dynamics need social psychology in addition to the
natural sciences we propose the application of three categories: phenomena,
behavior and action. They permit a clear discrimination between situations in
which minimal models from the natural sciences are appropriate and those in
which sociological and psychological concepts are needed.
  To demonstrate the necessity of this framework, an experiment in which a
large group of people (n=270) enters a concert hall through two different
spatial barrier structures is analyzed. These two structures correspond to
everyday situations such as boarding trains and access to immigration desks.
Methods from the natural and social sciences are applied. Firstly, physical
measurements show the influence of the spatial structure on the dynamics of the
entrance procedure. Density, waiting time and speed of progress show large
variations. Secondly, a questionnaire study (n=60) reveals how people perceive
and evaluate these entrance situations. Markedly different expectations, social
norms and strategies are associated with the two spatial structures. The
results from the questionnaire study do not always conform to objective
physical measures, indicating the limitations of models which are based on
objective physical measures alone and which neglect subjective perspectives.
