Secure safety monitoring is an intriguing and practical feature of current ride-hailing services. It enhances the riders’ safety after they are onboard a driver’s vehicle while not violating location privacy. Existing work suffer from the problems of unnecessary trajectory uploading, centralized trajectory monitoring, redundant trajectory monitoring, and assuming static trajectories. In this work, we propose a decentralized, secure, and efficient safety monitoring scheme Artemis to guarantee rider safety while supporting dynamic trajectories, i.e., permitted trajectory deviations. In specific, we design a 2-out-of-n private threshold signature protocol to achieve trajectory authenticity, design a decentralized RHS platform and a secure trajectory similarity computation protocol to guarantee both efficiency and location privacy, and design a secure three-party computation protocol to ensure deviation authenticity. Formal security experiments and proofs are provided. Experimental results from extensive experiments based on Ethereum and Intel SGX2 demonstrate that Artemis is highly efficient.