Personally they annoy me. I am sure they help keeping people behind you aware, but when I am behind one it just ticks me off. I see them on motorcycles a lot as well. I don't doubt they work well and probably significantly reduce rear end collisions, but they do irk me. If they were adjustable, with maybe 1 flash, then solid, I wouldn't mind as much. I remember seeing them on motorcycles where they flash faster based on how hard you were braking, those are very effective.