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The IEEE 802.11p wireless access in vehicular environment (WAVE) protocol providing for vehicle-to-infrastructure and vehicle-to-vehicle radio communication is currently under standardization. We provide an NS-2 simulation study of the proposed IEEE 802.11p MAC protocol focusing on vehicle-to-infrastructure communication. We show that the specified MAC parameters for this protocol can lead to undesired throughput performance because the backoff window sizes are not adaptive to dynamics in the numbers of vehicles attempting to communicate. We propose two solutions to this problem. One is a centralized approach where exact information about the number of concurrent transmitting vehicles is used to calculate the optimal window size, and the other is a distributed approach in which vehicles use local observations to adapt the window size.We show that these schemes can provide significant improvements over the standard MAC protocol under dense and dynamic conditions.