From: Jobst Brandt
Mirrors are mandatory on virtually every other type of vehicle on the road. Competent drivers iders learn the limitations of the information available from their mirrors and act accordingly.
I suppose the question is appropriate because no one seems to have a good explanation for this. In such an event, when there is much evidence that what would seem obvious is not what is practiced, I assume there are other things at work. I for one don't wear glasses to which to attach a mirror and putting it on a helmet seems a fragile location when the helmet is placed anywhere but on the head.
These are not the real reasons though, because I have found that when looking in a head mounted mirror, I cannot accurately tell anything about the following vehicle's position except that it is behind me. That is because I am looking into a mirror whose angular position with respect to the road is unknown. The rear view mirror in a car is fixed with respect to the direction of travel and objects seen in it are seen with reference to ones own vehicle, be that the rear window frame or side of the car. I find the image in a head mounted mirror on a bicycle to be distracting and a source of paranoia if I watch it enough. It does not tell me whether the upcoming car is, or is not, going to slice me.
I found this to be the case when I first started using a head-mounted mirror, but with time and experience, It ceased to be a problem.
Unlike Jobst, I very highly recommend helmet-mounted mirrors. I find them actually superior to automotive rear-view mirrors, because you can scan from side to side by turning your head, giving them a much wider effective field of view.
I additionally I find it difficult to focus on objects when my eyeballs are distorted by turning them as much as 45 degrees to the side of straight ahead. You can try this by reading these words with your head turned 45 degrees from the text.
I believe these two effects are the prime reasons for the unpopularity of such mirrors. They don't provide the function adequately and still require the rider to look back. I do not doubt that it is possible to rely on the mirror but it does not disprove my contention that the information seen is by no means equivalent to motor vehicle rear view mirrors to which these mirrors have been compared. It is not a valid comparison.
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