  The increasing number of mass events involving large crowds calls for a
better understanding of the dynamics of dense crowds. Inquiring into the
possibility of a mechanical description of these dynamics, we experimentally
study the crossing of dense static crowds by a cylindrical intruder, a
mechanical test which is classical for granular matter. The analysis of our
experiments reveals robust features in the crowds' response, comprising both
similarities and discrepancies with the response of granular media. Common
features include the presence of a depleted region behind the intruder and the
short-range character of the perturbation. On the other hand, unlike grains,
pedestrians anticipate the intruder's passage by moving much before contact and
their displacements are mostly lateral, hence not aligned with the forces
exerted by the intruder. Similar conclusions are reached when the intruder is
not a cylinder, but a single crossing pedestrian. Thus, our work shows that
pedestrian interactions even at high densities (3 to 6 ped/m 2) do not reduce
to mechanical ones. More generally, the avoidance strategies evidenced by our
findings question the incautious use of force models for dense crowds.
