In this paper we study the channel occupation induced by the OLSR proactive routing protocol used in a linear Vehicular Ad hoc Network (VANET). Unlike previous studies which usually use simulations to evaluate the overhead of routing protocols, we derive a simple analytical model to carry out this evaluation. Moreover, we do not evaluate the total overhead induced by the routing protocol as is usually proposed, but for a given node we compute the channel occupation induced by the routing protocol. This paper provides a quantitative approach to evaluating the cost of a proactive routing protocol in a linear vehicular ad hoc network as a function of several parameters such as the frequency of the control messages, the density of the vehicles, the propagation range of the control messages and the carrier sense area