This paper is about the application of formal methods to model and analyze complex systems in the context of Intelligent Transport Systems (ITS). It suggests a specification methodology based on a set of UML diagrams to generate a complete analyzable formal model. The methodology integrates the requirements of incremental and modular development for complex systems. The analysis made on the formal model is carried out through qualitative criteria, verified by model checking tools. The proposed guidelines are illustrated by a case study which considers cars in traffic situations, exchanging information about their states to reach consistency among their driving decisions.