  We propose a model for the intersection of two urban streets. The traffic
status of the crossroads is controlled by a set of traffic lights which
periodically switch to red and green with a total period of T. Two different
types of crossroads are discussed. The first one describes the intersection of
two one-way streets, while the second type models the intersection of a two-way
street with an one-way street. We assume that the vehicles approach the
crossroads with constant rates in time which are taken as the model parameters.
We optimize the traffic flow at the crossroads by minimizing the total waiting
time of the vehicles per cycle of the traffic light. This leads to the
determination of the optimum green-time allocated to each phase.
