  The fundamental diagram of pedestrian dynamics gives the relation between the
density and the flow within a specific enclosure. It is characterized by two
distinctive behaviors: the free-flow regime (for low densities) and the
congested regime (for high densities). In the former, the flow is an increasing
function of the density, while in the latter, the flow remains on hold or
decreases. In this work, we perform numerical simulations of the pilgrimage at
the entrance of the Jamaraat bridge (pedestrians walking along a straight
corridor) and compare flow-density measurements with empirical measurements
made by Helbing[1]. We show that under high density conditions, the basic
Social Force Model (SFM) does not completely handle the fundamental diagram
reported in empirical measurements. We use analytical techniques and numerical
simulations to prove that with an appropriate modification of the friction
coefficient (but sustaining the SFM) it is possible to attain behaviors which
are in qualitative agreement with the empirical data. Other authors have
already proposed a modification of the relaxation time in order to address this
problem. In this work, we unveil the fact that our approach is analogous to
theirs since both affect the same term of the reduced-in-units equation of
motion. We show how the friction modification affects the pedestrian clustering
structures throughout the transition from the free-flow regime to the congested
regime. We also show that the speed profile, normalized by width and maximum
velocity yields a universal behavior regardless of the corridor dimensions.
