One way to improve road safety for cyclists is the development of hazard notification systems. Instead of in field experiments, such systems could be tested in safe and more controlled simulated environments; however, their validity needs verification. We evaluated the validity of mixed reality (MR) simulation for bicycle support systems notifying of dooring hazards. In a mixed-design study (N=43) with environment type(MR/test track) as within and hazard notifications (with/without) as between factor, comparing subjective and objective measures across environments. In conclusion, MR simulation is absolutely valid for user experience and perceived safety and relatively valid for workload, standard deviation of lateral position, and speed. However, MR simulation was not valid for lateral distance, as participants cycled more in the center of the street than on the test track, perhaps to avoid simulator sickness. Thus, we conclude that MR simulation is valuable for studying bicycle safety.