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Recently, several techniques have been proposed to protect the user location privacy for location-based services in road network environments. A typical approach for user location privacy protection is to blur a user location into a cloaked set of road segments S such that S satisfies the user specified privacy requirements. However, these location cloaking algorithms work with snapshot locations only and do not consider the effect of continuous location updates. Applying these algorithms directly to continuous queries would lead to privacy leakage if attackers have knowledge about maximum user velocity, namely velocity-based Attack. In this paper, we present the privacy safety condition to defend against velocity-based attack, and propose an anonymity algorithm based on greedy strategy to preserve user privacy. The experiment results based on dataset of real network show the algorithm is effective and feasible.