It is well known that a time division multiple access (TDMA)-based medium access control (MAC) is advantageous in many aspects in vehicular ad hoc networks (VANETs). These include its ability to prevent the hidden-terminal problem, and the guarantee of strict quality-of-service (QoS) for providing real-time applications. However, time slot assignments to vehicles could suffer from an unstable problem, called merging collisions. This problem is due mainly to the changing network topology of a VANET, which can be characterized by vehicles joining into or leaving from a cluster of vehicles. In this paper, we focus on the problem of unstable time slot assignments facing several existing TDMA approaches, and a scheme is proposed to effectively alleviate this problem.