I had my mind set that I need some sort of an emergency respirator mask that I can use when I ride through very long tunnels, or extremely busy streets where motor vehicles just make it difficult to breathe, as well as for those foggy-smoggy winter days.
I have the Zefal Spy Mirror mounted on the most left edge of my handle bars, bellow the bars. It is left, since in my country, like in Europe, the driving is done on the right side of the road. I purchased it a year ago thinking that it may come useful on long tours on frequent roads. I thought that I could check for cars when turning left, but also for any issues when I'd like to stop, because I sure do not want to have another bicycle rider or even a car hitting me in the rear.