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Vehicular ad hoc networks (VANETs) are designed to improve traffic safety and efficiency. To this end, the traffic communication must be authenticated to guarantee trustworthiness for guiding drivers and establishing liability in case of traffic accident investigation. Cryptographic authentication techniques have been extensively exploited to secure VANETs. Applying cryptographic authentication techniques such as digital signatures raises challenges to efficiently store signatures on messages growing with time. To alleviate the conflict between traffic liability investigation and limited storage capacity in vehicles, this paper proposes to aggregate signatures in VANETs. Our proposal can preserve privacy for honest vehicles and trace misbehaving ones, and provides a practical balance between security and privacy in VANETs. With our proposal, cryptographic witnesses of safety-related traffic messages can be significantly compressed so that they can be stored for a long period for liability investigation. Our proposal allows a large number of traffic messages to be verified as if they were a single one, which speeds up the response of vehicles to traffic reports.