  Motivated by aggregation phenomena in gliding bacteria, we study collective
motion in a twodimensional model of active, self-propelled rods interacting
through volume exclusion. In simulations with individual particles, we find
that particle clustering is facilitated by a sufficiently large packing
fraction (eta) or length-to-width ratio (kappa). The transition to clustering
in simulations is well captured by a mean-field model for the cluster size
distribution, which predicts that the transition values kappa_c of the aspect
ratio for a fixed packing fraction is given by kappa_c = C/eta - 1 where C is a
constant.
