Driving down Second Ave. near the East 70s in New York City one evening while trying to find a parking place, I nearly creamed two cyclists who were:
-riding the wrong way up the left hand side of the street (a major thoroughfare)
-with no lights or reflectors
-with no helmets
-wearing all black

They were almost impossible to see against the visual blur of store signs, street lights, car lamps, etc. Had I been going any faster I could not have reacted fast enough and would have hit them.

Why do people do stupid things like this?