OK, I just got back from a 28 mile ride out to Audubon, PA and back home on the local MUP.
Every bike I passed... all bikes going in my direction... the people with mirrors seemed to be completely clueless, and seemed to be completely surprised when I passed, even if I used my bell and shouted that I was passing on the left. The people without mirrors typically made eye contact with me several times during the 100 yards when I was approaching them and overtaking them, and never broke their line.
The burning question:
Do mirrors cause people to be (or become) clueless, or is it that clueless people gravitate toward using mirrors?