The Biham-Middleton-Levine traffic model is a simple system that demonstrates phase transitions and self-organization. It is the basis for extensive modern studies of traffic flow. Depending on the initial density, the system shows a sharp phase transition from free-flowing to completely congested traffic, but also stable intermediate stages in which traffic jams and free-flowing traffic coexist. The geometric structure of such phases is very regular, with bands of free-flowing traffic meeting at blocked wave fronts that propagate uniformly through space.