Privacy-Preserving Traffic Monitoring in Vehicular Ad Hoc Networks Most modern metropolitan cities have suffered from increasing traffic accidents and jams. Vehicular adhoc network (VANET), consisting of information collecting, processing and transmitting units embedded in vehicles assisted by roadside infrastructures, has been proposed as one the most promising solution to problems introduced by the increasing number of vehicles in modern cities. To achieve the goal, it is crucial to allow the transportation administration center to collect information about the traffic and road status through VANET. A major obstacle in this scenario is the privacy concern on the vehicles. To address this issue, this paper proposes a generic privacy-preserving traffic monitoring framework which allows individual vehicle driving status and the road usage information are collected while the privacy of the vehicles is well preserved. This goal is achieved by the novel technology of distinguishing individual vehicles with their spatio-temporal occupations. The continual change of spatio-temporal identities provides privacy for vehicles in a natural way, which remains nonetheless traceable by a trusted authority to prevent misbehaving vehicles from abuse the privacy-preserving mechanism provided by the system.