A social force model for the crowd evacuation in a terrorist attack

A social force model is developed in this paper to study the crowd evacuation when a terrorist attack occurs in the public place. The persons in the model are divided into two groups—terrorists and pedestrians. On one hand, each terrorist chooses the nearest pedestrian as the target. Once the distance between him and his target is small enough, the terrorist will launch attacks on his target and the target will be killed with a certain probability. On the other hand, each pedestrian tries to avoid the terrorists and reach the exits. An emergency exit choice strategy which emphasizes the security risk factor is developed for pedestrians. The main simulation results are summarized as follows. First, the number of deaths in the terrorist attack increases with the number of terrorists and with the density of pedestrians. If the number of exits decreases, the death toll will become more sensitive to the change of the density of pedestrians. Second, adding the number of exits can significantly reduce casualties. Third, more pedestrians will be killed and the evacuation speed will be reduced if terrorists start the attack from the positions of the exits. Fourth, the emergency exit choice strategy has an advantage over the ordinary exit choice strategy in daily life for reducing casualties. The more unbalanced the terrorists’ initial distribution around the exits is, the more noticeable this advantage will be. Fifth, the number of deaths will decrease obviously if at least half of the exits are available and safe. Our study is valuable for developing an effective evacuation scheme to reduce casualties in a terrorist attack.