—Several contention-based Medium Access Control (MAC) protocols have been proposed for the vehicles to gain the radio channels to distribute active safety messages timely, e.g., Safety Critical Application (SCA) information, for inter-vehicle communications in Vehicular Ad Hoc Networks (VANETs). In these MAC protocols, a contention period is introduced before the vehicle for channel access so that we may not have the timely and reliably message dissemination in VANETs. To reduce the contention period, this paper proposes a Region-based Clustering Mechanism (RCM) to be applied in these MAC protocols. We propose analytical models to investigate the performance of the RCM, which are validated by the simulation experiments.