I don't think a mirror is necessarily safer on a busy highway with high speed traffic and a narrow shoulder. In this type of situation you are unlikely to have the warning to get out of the way unless you are spending too much time looking at the mirror and not enough on the road ahead.
However I do believe mirrors are invaluable in awareness on the road. I use a helmet mounted mirror when riding an upright and a bar mounted mirror when on my recumbent. The mirror makes it much easier to be aware of approaching traffic. I can know when it is safe to move to the left to avoid obstacles such as parked cars, debris, potholes, storm grates, etc. It also makes changing lanes easier when you need to make a left hand turn.
Craig