Bad drivers - which, unfortunately, seem to make up a very large percentage of the Great British Public that managed, somehow, to pass their driving tests.
One twat actually tried to barge me off the motorway sideways a few weeks ago, late at night, as he wasn't impressed that I'd applied my brakes to get him removed from my backside, where he was lodged. After I'd finished overtaking all the cars I was in the process or overtaking (I wasn't travelling slowly either), I moved into the middle lane and he drew up alongside me, and swerved violently into my lane, forcing me to swerve violently into the inside lane, where there was a car not far behind. Absolute tosser. And if he's reading this, I took down your registration plate, arsehole.