Since the drive-thru Internet service becomes one of promising vehicular applications, it can be easily affected by the problem of system overload when unpredictable traffic of request and response workloads exceeds the service capacity that a roadside unit can stand for. To mitigate this situation, this paper proposes an efficient data aggregation mechanism which is able to perform request fusion and response diffusion among vehicles to reduce message traffic in the network. Functionally, a roadside unit can delegate relay vehicles to merge requests from neighboring vehicles into a compound request, and later reply with a batch of response messages. Relay vehicles then deliver responses to requesting vehicles in proximity in a broadcasting manner. By simulation results, the effort of this proposed data aggregation mechanism can significantly reduce message traffic. This mechanism can defer the critical time of system overload to some extent and meanwhile moderate the service starvation situation under heavy request workload.