I at least try to never assume that someone is stupid and always make any assessment of others from a assumption of competency. So when I see someone riding... questionably, I simply assume that they know better and choose not to act better.
The only caveat to that is I would tell someone if their helmet was on backwards. I do not consider myself a superior being, so I have no desire to lecture grown adults on how to behave on a bicycle. We can know right from wrong and still do wrong-- that's free will, baby.
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