  We investigate the appearance of trapping states in pedestrian flows through
bottlenecks as a result of the interplay between the geometry of the system and
the microscopic stochastic dynamics. We model the flow trough a bottleneck via
a Zero Range Process on a one dimensional periodic lattice. Particle are
removed from the lattice sites with rates proportional to the local occupation
numbers. The bottleneck is modelled by a particular site of the lattice where
the updating rate saturates to a constant value as soon as the local occupation
number exceeds a fixed threshold. We show that, for any finite value of such
threshold, the stationary particle current saturates to the limiting bottleneck
rate when the total particle density in the system exceeds the bottleneck rate
itself.
