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Inter-Vehicular communication is considered to be a promising way to improve traffic safety. Among this, it is a key technology to delivery urgent messages and periodical beacons to neighbors successfully. In this paper, we propose an Adaptive Collision-Free MAC (ACFM) protocol tailored to vehicular networks, based on dynamic TDMA mechanism. In our protocol, RoadSide Units (RSUs) are exploited to schedule time slots assignment and disseminate control information, while the data are spread among vehicles. Each RSU maintains a dynamical slots assignment cycle for vehicles in its coverage adaptively. A cycle consists of N frames, where N varies from 1 to 5 according to the number of vehicles. Under the scenario of light traffic, ACFM controls the excessive increase of unassigned slots through shrinking slots assignment cycle frame by frame. When there are a mass of vehicles on roads, ACFM provides …