Research in vehicular ad-hoc networks (VANETs) have attracted a lot of attention in the recent years as emerging wireless technologies have opened up the way to many new exciting applications. VANETs are highly dynamic wireless networks that are designed to support vehicular safety, traffic management, and user-oriented applications. Each vehicle can exchange information to inform other vehicles about the current status or a dangerous situation such as an accident. Detecting and sending information about such situations requires a reliable broadcast service between vehicles, thus increasing the need for an efficient medium access control (MAC) protocol. In this paper, we propose ASAS, an Adaptive Slot Assignment Strategy, which takes advantage of bandwidth spatial reuse and reduces intra-cluster and inter-cluster message collisions without having to use an expensive spectrum and complex mechanisms such as CDMA or FDMA. Cluster heads (CHs) which are elected among the vehicles are then responsible for assigning time slots to the other vehicles in their clusters. The evaluation results show the interest of ASAS in terms of slot reuse and collision rates in different speed conditions