  It has been shown that placing an obstacle in front of an exit door has
proven to be a successful method to improve pedestrian evacuations. In this
work, we will focus on the space limited by the exit and the obstacles (i.e.
the vestibule structure). We analyzed two different types of vestibules: the
two-entry vestibule (which consists of a single panel-like obstacle) and the
three-entry vestibule (which consists of two panel-like obstacles). In the
former, we studied the effects of varying the walls' friction coefficient
$\kappa_w$ and the distance from the obstacle to the exit door $d$. In the
latter, we varied the space between the two panels (gap). We found that the
three above mentioned parameters control the vestibule's density, which
subsequently affects the evacuation flow (fundamental diagram). We have also
found that reducing the distance $d$ or increasing the friction facilitates the
formation of blocking clusters at the vestibule entries, and hence, diminishes
the density. If the density is too large or too low, the evacuation flow is
suboptimal, whereas if the density is around $2 \pm 2$, the flow is maximized.
Our most important result is that the density (and therefore the evacuation
flow) can be precisely controlled by $\kappa_w$, $d$, and the gap. Moreover,
the three-entry vestibule produced the highest evacuation flow for specific
configurations of the gap and the distance from the panels to the door.
