When I was cycling coast to coast I recall another rider announcing one evening at dinner that while he was crossing the road, he was nearly struck by a passing car because he didn’t see the car in his handlebar mirror. He went on to advocate that other riders with handlebar mirrors should immediately remove them for their own safety.
I’m not sharing this story to discourage the use of handlebar mirrors. In fact, I prefer to ride with a handlebar mirror. However, this story does illustrate the danger of riding without understanding your mirror’s field of view limitations. The field of view is the extent of the observable world that can be seen. Not understanding your bicycle mirror’s field of view can have disastrous results.
A mirror can’t display everything beside and behind you. Although I can’t find any information on the field of view of a bicycle mirror, there are published reports about the field of view of car side view mirrors. One report is the Field of View in Passenger Car Mirrors by the University of Michigan Transportation Research Institute that found the mean horizontal field of view width was 12.9 degrees for the left (driver-side).
Compared to a human’s total field of view which is between 160 to 208 degrees, a mirror can only provide a very small slice of what is beside or behind you. As a result, a mirror must be carefully aimed and used with a clear understanding of what can and can’t be seen with it. In addition, never move into traffic or cross lanes without turning your head to glance back and get a better view of what is next to and behind you.