This paper proposes a modified version of vehicular ad hoc network media access control (VeMAC) protocol for further increased throughput. In the modified VeMAC, an additional bit is appended to the field for each time slot in the header used in VeMAC as the collision flag. Upon detecting a collision for a time slot, a node sets the collision flag for that time slot without changing the ID field and sends its packet in its acquired time slot. By receiving the packet with the collision flag, nodes notice a collision happen in the time slot. One of the nodes involved in the collision knows from its ID field that it is the node itself that is supposed to use the time slot, and therefore it continues to use the time slot.
The other nodes recognize themselves as nodes that should not use the time slot because their IDs are not included in its ID field of the packet. Consequently, they release the time slot and begin by acquiring new time slots. Since not all nodes involved in the collision release the time slot in our modified VeMAC, the possibility that access collisions following the merging collision happen is reduced, compared to the original one. Simulation results show that our modified one outperforms the original one in terms of the average number of collisions and throughput.