When driving, what gets the most in your nerves?

Apart from people who don't use turn signals properly or at all (watch a roundabout for five minutes and you'll see what I mean), people who don't understand when to give way to people coming the opposite direction, and hold everyone else up in the process. Here in England you're supposed to give way to traffic approaching your right (driver) side at a roundabout or junction, but you see so many pricks who just stop there gawking while a huge line forms behind them. Also, if there's an obstruction (such as a parked car) on your side, you pull up behind it and give way to oncoming vehicles, you don't gun it and see if you can get past before that eighteen-wheeler smashes up your car. That last one probably doesn't happen as much in the US where the roads are generally wider/better than ours, but here it just winds me up.

Jam it back in, in the dark.