Road traffic microsimulations based on the individual motion of all vehicles are now recognized as an important tool to describe, understand and manage road traffic. Cellular automata models are a very efficient way to implement car motion. This paper presents a detailed description of a parallel cellular automata traffic microsimulator. We discuss the data structure, domain decomposition, and provide a detailed performance analysis for a problem of size n on p processors. We consider realistic simulation for the city of Geneva and report some results concerning the global traffic behavior. Provided that enough processors are used, large scale simulation can be performed in less time than the real process.