the friction parameter $\mu$ for a room with different configurations of the column.

![Evacuation time as function of the friction parameter $\mu$ for four room geometries: no column, central column and column shifted by one and two lattice sites. The initial density is $\rho=0.3$ and the coupling strengths are $k_S=10$, $k_D=0$ corresponding to the ordered regime.[]{data-label="plot_16"}](\DIR/fig26.eps){width="0.8\columnwidth"}

The placing of the column does not influence the evacuation times for $\mu=0$. Here the movement is nearly deterministic and the particles do not hinder each other. For higher values of $\mu$ the column becomes more and more relevant. It causes a subdivision of the crowd and decreases the local pressure between the particles. Therefore the evacuation times become smaller. They are minimized for a column placed in a slightly asymmetric configuration in front of the exit (with a shift of one lattice site, fig. \[plot\_16\]).

How can this surprising result be understood? On one hand, the column has a certain screening effect that forces some pedestrians to take a detour and therefore potentially increases the evacuation time. On the other hand, the column subdivides the pedestrian flow and so can lead to a reduction of conflict situations, especially close to the exit. The 