  We report on two series of experiments, conducted in the frame of two
different collaborations designed to study how pedestrians adapt their
trajectories and velocities in groups or crowds. Strong emphasis is put on the
motivations for the chosen protocols and the experimental implementation. The
first series deals with pattern formation, interactions between pedestrians,
and decision-making in pedestrian groups at low to medium densities. In
particular, we show how pedestrians adapt their headways in single-file motion
depending on the (prescribed) leader's velocity. The second series of
experiments focuses on static crowds at higher densities, a situation that can
be critical in real life and in which the pedestrians' choices of motion are
strongly constrained sterically. More precisely, we study the crowd's response
to its crossing by a pedestrian or a cylindrical obstacle of 74cm in diameter.
In the latter case, for a moderately dense crowd, we observe displacements that
quickly decay with the minimal distance to the obstacle, over a lengthscale of
the order of the meter.
