Since vehicular safety applications have strict requirements on reliability and low latency, we propose a novel medium access control protocol (MAC) for VANETs. Cluster heads are elected based on their relative speed and stability on the road with minimal overhead since all clustering information are embedded in control channel's safety messages. We showed that the proposed protocol can support the traffic safety efficiency and reliability and increase the stability of the cluster topology by increasing the cluster head's lifetime and the dwell time of its members.