Our main goal is to provide the best management protocols for vehicular Ad Hoc network (VANET), more precisely, to determine the best routing protocol. In this paper, we focus on VANET built on highway where cars run in the same direction. We introduce a quantitative model in order to evaluate existing routing protocols. We use this model to compare two main classes of routing protocols, topological protocols and geographic protocols. The studied criterium is the scalability property, i.e. performance preservation in spite of the substantially increasing of network size.