Cellular automaton model — stop-and-go waves emerge spontaneously
Nagel-Schreckenberg model (1992): each time step — (1) accelerate: v→min(v+1, v_max); (2) brake: v→min(v, gap-1); (3) dawdle: v→max(v-1,0) with probability p; (4) move. Despite purely local rules, phantom traffic jams (stop-and-go waves) emerge at intermediate densities. Space-time diagram shows backward-propagating jam fronts.