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Due to the mobile nature of nodes comprising the vehicular networks in addition to interference and congested network traffic, the design of an efficient MAC network becomes challenging. Moreover, the latency constraints underlying safety application add complexity to this design. The TXOP (Transmit Opportunity) mechanism defined in the IEEE 802.11e is optimized to meet the requirements of a Multimedia Network (voice and audio) but not a vehicular network thus making it unsuitable to be used in a VANet environment. On the other hand, existing MAC algorithms for VANets are designed to overcome some challenges overlooking others. Hence, the merit of this paper lies in developing a MAC mechanism that incorporates in an integrated manner various VANet's challenges while modifying the 802.11e TXOP to meet vehicular conditions. In this work, we propose MoByToP (Mobility Based Dynamic TXOP for VANets). MoByToP is an enhancement mechanism added to the MAC layer to extend the operation of the medium reservation phase. The uniqueness of MoByToP lies in designing a dynamic TXOP to incorporate mobility and QoS (Quality of Service) while considering the transmit data rate of the source vehicle. The proposed protocol was implemented in OMNET++4.1 and extensive experiments demonstrated that the proposed MAC with MoByToP mechanism ensures the reception of QoS messages much faster, more efficient and reliable than existing VANet MAC protocols.