Originally Posted by Helmet Head
That's the convenience factor that he referred to is, and he's fine with it.
Certainly if you're thinking of a line of traffic passing you by, and the possibility that anyone in that line might suddenly drift into you out of the line, yes, that's impossible to detect in time.
Of course, that's not at all what I'm talking about.
I'm talking about the situation where there is no other same direction traffic at the moment, and someone is coming. I think you can do a lot to determine if that particular driver has noticed you. Further, I think a driver in that situation is about the only kind of driver who will drift and hit a cyclist, so that's where the focus needs to be, if that's what you're looking for.
I still don't buy your latter use, honestly. I toured for years and did not have a mirror most of the time then... and even now when I am out on quiet roads where those long gaps do exist... I can't determine enough about an approaching motorist from any distance with a mirror to make any difference. Usually I hear them long before I might "in a glance" notice them... and I certainly cannot determine anything such as "did they see me" for any perceivable distance... especially when they are moving over 50MPH.