You know something else that really peeves me? Not because it's one of the worst things people do but because of the amount of ever-increasing frequency it occurs.
When I'm on a busy road & I see a car in the other lane wanting to change into mine, where possible, I always let them in. It really shits me when the drive in & ignore me. It doesn't take much effort to offer a "thank you" wave. I ALWAYS acknowledge the other driver when s/he lets me in & when I let someone in who does wave, I always give a quick "you're welcome" wave in return.
When you let a complete stranger in front of you in a queue, you wouldn't expect them to just step in & ignore you so why should the situation be any different when you're in a car? We still living in a society & doesn't living in a society mean having some manors? Do people leave their manors at home when they get in the car?
I don't have much time for really selfish & self-centred people...