A cellular-automata model of flow in ant-trails: non-monotonic variation   of speed with density
  Generically, in models of driven interacting particles the average speed of the particles decreases monotonically with increasing density. We propose a counter-example, motivated by the motion of ants in a trail, where the average speed of the particles varies {\it non-monotonically} with their density because of the coupling of their dynamics with another dynamical variable. These results, in principle, can be tested experimentally. 
Debashish Chowdhury, Vishwesha Guttal, Katsuhiro Nishinari and Andreas   Schadschneider