When a group of people (on foot or in cars) move very slowly in a straight line, either side by side or one in front of the other, holding you up in some way or another.
Dammit, I'm late for a foot appointment and I need to cross this intersection, but these three cars are on a retard parade so I have to wait until the red light!
When your IQ temporarily drops an extreme amount and the sound of a parade plays in your mind. Usually caused by extreme joy and/or surprise.
Dude....my mind just went on a retard parade...