Vehicular communications are considered the next step in increasing transportation safety and comfort. The prohibitive price of operational test beds means that computer simulations are the only viable solution for analysing the performance of different protocols and architectures. However, simulation frameworks used in vehicular ad-hoc networks research are still highly heterogeneous and, as a consequence, many of the proposed ideas can not be compared and validated. In this paper, we focus on the challenges faced when modelling the vehicular environment and the solutions adopted in the main simulation tools. As the research community is concerned with many different problems, from safety related issues to traffic efficiency and from intersection management to Internet access, we consider that every study should chose the appropriate simulator based on its requirements. Consequently, we make some recommendations which take into account the scope of the simulated scenario and the properties of the simulation frameworks.