Sure signs someone can't drive

After much scientific research, I have developed a working theory that one's ability to operate a motor vehicle is directly correlated with the presence or absence of two forms of auto decoration. If you ever get behind a car with a Jesus fish, watch out, that person is going to vary their speed dramatically, hit the brakes at inopportune times, and cut you off as though you don't exist. It's a fact. Similarly if a person has the little flag banner hanging from the rear-view mirror. Be prepared, that person is going to come from behind you, pass you, cut you off only to turn at the next intersection, they most likely will be playing excessively loud music that drowns out your own stereo, and they will either be driving very aggressively, or very very conservatively(to avoid raising suspicion?) Supporting evidence will follow as I perform more observations. Grumpmeter is at level 4 today.