Case I: evacuation time of fifty trials with different pedestrian densities. (a) Each dot represents one trial. Once there was an additional 100 pedestrians, 50 trials would be simulated in each condition. (b) The additional time to evacuate the first 100 pedestrians was 0. The other lines represent the additional average evacuation time for an additional 100 pedestrians compared with that in the previous stage. 200 pedestrians and 900 pedestrians in panel (b) represent the most efficient evacuation time in low and high pedestrian density respectively.