Electronic Toll Pricing (ETP), a location-based vehicular service, allows users to pay tolls without stopping or even slowing down their cars. User location records are collected so as to calculate their payments. However, users have privacy concerns as locations are considered as private information. In this paper, we focus on user traceability in ETP systems where anonymous location records are stored by the service providers. Based on user toll payment information, we propose a post-hoc analysis of user traceability, which aims at computing a user's all possible traces.Moreover, we propose several methods to improve the effectiveness of the analysis by combining other contextual information and propose a number of optimisations to improve its efficiency as well. We develop a prototype and evaluate the effectiveness of the analysis by conducting extensive experiments on a number of simulated datasets.
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