  In human crowds, interactions among individuals give rise to a variety of
self-organized collective motions that help the group to effectively solve the
problem of coordination. However, it is still not known exactly how humans
adjust their behavior locally, nor what are the direct consequences on the
emergent organization. One of the underlying mechanisms of adjusting individual
motions is the stepping dynamics. In this paper, we present first quantitative
analysis on the stepping behavior in a one-dimensional pedestrian flow studied
under controlled laboratory conditions. We find that the step length is
proportional to the velocity of the pedestrian, and is directly related to the
space available in front of him, while the variations of the step duration are
much smaller. This is in contrast with locomotion studies performed on isolated
pedestrians and shows that the local density has a direct influence on the
stepping characteristics. Furthermore, we study the phenomena of
synchronization -walking in lockstep- and show its dependence on flow
densities. We show that the synchronization of steps is particularly important
at high densities, which has direct impact on the studies of optimizing
pedestrians flow in congested situations. However, small synchronization and
antisynchronization effects are found also at very low densities, for which no
steric constraints exist between successive pedestrians, showing the natural
tendency to synchronize according to perceived visual signals.
