  We study two new models of two particle species invading a surface from
opposite sides. Collisions of particles of different species lead to the
formation of congestion fronts. One of the models implements a reversible
process whereas in the other model the congestion front forms irreversibly. For
both models we find that the congestion fronts are self-affine but with
different roughness exponents. For low densities the system does not congest
and we find a phase transition between a phase of freely moving particles and a
congestion phase.
