The use of intelligent transportation systems (ITSs) is expanding, and vehicle-to-vehicle communication (V2V) and vehicle-to-infrastructure communication (V2I) have attracted considerable attention. During disasters, it becomes difficult to share information with vehicles in sparse areas and distant places. Therefore, we considered using unmanned aerial vehicle (UAVs) in ITSs during disasters. We developed a method to control the flight speed and modulation and coding scheme (MCS) of a UAV and maximize the number of vehicles that receive data in one transmission. A model to derive the optimal UAV's flight speed and MCS was constructed. Then, we confirmed that an optimal UAV's flight speed and MCS exist that maximize the number of vehicles receiving data in one transmission by the UAV by numerical analysis. In the evaluation using the flight speed and MCS derived from the model, the expected number of vehicles that received data in one transmission at each data size held by the UAV was derived and compared with other methods. From the evaluation, we confirmed that the proposed method maximizes the number of vehicles receiving data in one transmission by the UAV for each data size maintained by the UAV.