Autonomous vehicles are complex safety critical systems that operate in uncertain and dynamic environment. To ensure safe behavior, possibly at all times in all possible situations, they must be aware of themselves, their environment and take decisions accordingly. To this end, safety monitoring is a valuable technique that allows the vehicle to observe its behavior and trigger corrective measures in case of any violations. In this paper, we present a systematic specification for such a safety monitor. The monitor is a collection of safety rules that are obtained by performing hazard and risk analysis based on the operational mode and environmental situation of the vehicle at the time of the hazard. The rules act like safety constraints for the vehicle and in case of a violation generate a trigger to bring vehicle back to a safe state.