I agree, hold your line but make escape plans. there may be a true psycopath or two out there, but all but a tiny fraction are just idiots.
If it happened a lot, I guess I would A) ride with video (I do anyway), and B) start carrying a few 1" diameter steel ball bearings in a bar bag. One of those flipped in front of a windscreen should properly express my displeasure.
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Work: the 8 hours that separates bike rides.