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Vehicular networks are emerging technology to provide efficient communications between mobile vehicles and fixed roadside units (RSU), and enable mobile multimedia applications and safety services with diverse quality of service (QoS) requirements. In this paper, we propose a busy tone based medium access control (MAC) protocol with enhanced QoS provisioning for life critical emergency services. By using busy tone signals for efficient channel preemption in both contention period (CP) and contention free period (CFP), emergency users can access the wireless channel with strict priority when they compete with multimedia users, and thus achieve the minimal access delay. Through efficient transmission coordination in the busy tone channel, contention level can be effectively reduced, and the overall network resource utilization can be improved accordingly. We then develop an analytical model to quantify the channel access delay of emergency messages. Extensive simulations with Network Simulator (NS)-2 validate the analysis and demonstrate that the proposed MAC can guarantee reliable and timely emergency message dissemination in a vehicular network.