Recent advances in wireless network technologies, like the ability to operate in a high-speed mobility environment, have opened up to a number of domains such as that of Intelligent Transport Systems (ITS) whose main component are so-called Vehicular Ad hoc Networks (VANETs). VANETs are associated with two kind of deployments: vehicular to Infrastructure (V2I), managing communications between vehicles and a fixed infrastructure, and Vehicular to Vehicular (V2V), for communications between vehicles. Since VANETs obey to the principles of mobile ad hoc networks, hence connections are established in a spontaneous and distributed manner, they suffer from lack of Quality of service guarantees as the access is not bounded. Modeling of VANETs is a complex task, as there are a number of factors that negatively affect the state-space dimension (i.e. the number of considered vehicles, the considered traffic, the mobility management). In this paper we introduce a formal model, in terms of a Markov chain, of the IEEE 802.11p MAC protocol, the basic access control mechanism employed in VANETs. Such model is the basis for computing relevant performance measures of the VANETs.