  An evacuation process is simulated within the Social Force Model. Thousand
pedestrians are leaving a room by one exit. We investigate the stationarity of
the distribution of time lags between instants when two successive pedestrians
cross the exit. The exponential tail of the distribution is shown to gradually
vanish. Taking fluctuations apart, the time lags decrease in time till there
are only about 50 pedestrians in the room, then they start to increase. This
suggests that at the last stage the flow is laminar. In the first stage,
clogging events slow the evacuation down. As they are more likely for larger
crowds, the flow is not stationary. The data are investigated with detrended
fluctuation analysis.
