  When two active Brownian particles collide, they slide along each other until
they can continue their free motion. For persistence lengths much larger than
the particle diameter, the directors do not change, but the collision can be
modeled as producing a net displacement on the particles compared to their free
motion in the absence of the encounter. With these elements, a
Boltzmann--Enskog-like kinetic theory is built. A linear stability analysis of
the homogeneous state predicts a density instability resulting from the
effective velocity reduction of tagged particles predicted by the theory.
