  There have been several highway traffic models proposed based on cellular
automata. The simplest one is elementary cellular automaton rule 184. We extend
this model to city traffic with cellular automata coupled at intersections
using only rules 184, 252, and 136. The simplicity of the model offers a clear
understanding of the main properties of city traffic and its phase transitions.
  We use the proposed model to compare two methods for coordinating traffic
lights: a green-wave method that tries to optimize phases according to expected
flows and a self-organizing method that adapts to the current traffic
conditions. The self-organizing method delivers considerable improvements over
the green-wave method. For low densities, the self-organizing method promotes
the formation and coordination of platoons that flow freely in four directions,
i.e. with a maximum velocity and no stops. For medium densities, the method
allows a constant usage of the intersections, exploiting their maximum flux
capacity. For high densities, the method prevents gridlocks and promotes the
formation and coordination of "free-spaces" that flow in the opposite direction
of traffic.
