Motion of people and cars

To a first approximation crowds of people seem to show aggregate fluid-like behavior similar to what is seen in gases. Fronts of people—as occur in riots or infantry battles—seem to show instabilities perhaps analogous to those in fluids. Road traffic that is constrained to travel along a line exhibits stop-start instabilities when its overall rate is reduced, say by an obstruction. This appears to be a consequence of the delay before one driver responds to changes in speed of cars in front of them. Fairly accurate cellular automaton models of this phenomenon were developed in the early 1990s.