The other day, I was following behind this vehicle as you describe, had my son with me at this time. This guy did the exact same thing as you described; he drives real slow like a lane hog in the fast lane. Then, he gradually drives slower and slower, for no damn reason, and then he slammed on his brakes. He's real lucky I had my son with me, otherwise he would've found out what asphalt feels like. Guy's a menace to society.
2 of 3 brake lights are out