In this article, we focus on vehicular ad hoc networks. In such networks, sensor data are usually exchanged between vehicles to warn or inform the drivers when an event is detected (e.g., accident, emergency braking, available parking space, etc.). In this article, we propose to aggregate and store these data in order to have a history of past events. We therefore use Flajolet-Martin sketches. Our goal is here to generate additional knowledge to assist drivers even if no event is detected