  A modified lattice gas model is proposed to study pedestrian evacuation from
a single room. The payoff matrix in this model represents the complicated
interactions between selfish individuals, and the mean force imposed on an
individual is given by considering the impacts of neighborhood payoff, walls,
and defector herding. Each passer-by moves to his selected location according
to the Fermi function, and the average velocity of pedestrian flow is defined
as a function of the motion rule. Two pedestrian types are included:
cooperators, who adhere to the evacuation instructions; and defectors, who
ignore the rules and act individually. It is observed that the escape time
increases with the panic level, and the system remains smooth for a low panic
level, but exhibits three stages for a high panic level. We prove that the
panic level determines the dynamics of this system, and the initial density of
cooperators has a negligible impact. The system experiences three phases, a
single phase of cooperator, a mixed two-phase pedestrian, and a single phase of
defector sequentially as the panic level upgrades. The phase transition has
been proven basically robust to the changes of empty site contribution, wall's
pressure, and noise amplitude in the motion rule. It is further shown that
pedestrians derive the greatest benefit from overall cooperation, but are
trapped in the worst situation if they are all defectors.
