VESPA (Vehicular Event Sharing with a mobile P2P Architecture) 1 is a system for enabling vehicles to share information in vehicular ad-hoc networks (VANETs). The originality of VESPA is to process and disseminate any type of event (e.g., available parking spaces, accidents, emergency braking, information relative to the coordination of vehicles in emergency situations, etc.). The basic functions of VESPA are both disseminating events to potentially interested vehicles and evaluating their relevance once received in order to determine, for instance, whether the driver should be warned or not. This paper concentrates on knowledge extraction in VESPA. In particular, it focusses on how to exploit data exchanged among vehicles to produce knowledge to be used later on by drivers. Existing systems only use exchanged data to produce warnings for drivers when needed. Then, data is considered obsolete and is deleted. In contrast, we propose to aggregate data once it becomes “obsolete”. Our...