Road critical situations like vehicles merging in a highway require a tight coordination between car drivers within a short time while vehicles are driving at high velocity. Any driver misbehavior or error may result in a car accident. Car communication offering information exchange in timely manner may drastically help drivers to accomplish this task and avoid incidents. Unfortunately, communication standard like 802.11p are not well dimensioned to guarantee the requirement of this kind of data exchange. In this paper we propose to use the Transmit And Reserve (TAR), an ad-hoc medium access protocol, for vehicular communications. We provide a study to demonstrate its adaptivity and short time fairness with regard to different highway car traffic conditions and we focus on a typical use case of a highway ramp car merge to show its efficiency