Data collection in Vehicular Ad-hoc Networks (VANets) is considered as an interesting application which aims at delivering road traffic, environmental information and other kinds of data to interested entities. The protocols developed for the wireless mobile networks and particularly for WSN cannot be applied directly to vehicular networks due to the high mobility and the frequent disconnection especially in the highway. Therefore, the design of an efficient protocol is depending firstly on the use of an efficient technique for medium access and secondly on a correct data delivery. In this paper, we propose a data collection protocol based on clustering and on he use of a dynamic medium access technique. This protocol is based on acknowledged delivery and allows retransmission of data in the case of loss or errors. We present simulation results which show the feasibility, the effectiveness and the robustness of our proposed protocol.