It's that why people tie a red bandana on items hanging out of their pickup's bed? or does it have to be a damn flag, as you put it?
Anyhow, I am always considerate.. it's unlikely that a bumper of a car or SUV would be 4 ft. above ground level, but I guess I monster truck would be... if someone hits me from behind, it's not likely that LEO would give me a ticket for not having a danm flag hanging off my bicycle... it's kinda hard to miss a contraption like mine on the road.