The best ones are truckers and cyclists around here. My all time favourite goes to the guy driving a tanker full of god only knows what:
The story is, I was riding a bus at the time, on the top deck. Up there, you can see into truck cabs. The bus was going a bit faster than the tanker, so it started overtaking it.
There were hazard stickers literally everywhere on the tank part. And they all read along the lines of: "Ridiculously flammable contents; this tanker is really just a huge-ass bomb on wheels; don't even THINK of fire when near this motherfucker!". Y'know, the kind of thing that makes you wonder just what the hell you're slowly going past.
So the bus slowly creeps up past this tanker, and my seat gets level with the cab. I look in, and the driver's got no hands on the wheel. This alone would've been stupid enough to qualify for story material, but what makes this guy the no.1 idiot I've ever seen on any roads ever was that he had no hands on the wheel, because he had a cigarette in one hand... and the lighter in the other.
Blank disbelief didn't really describe it. To say that I lost faith in humanity that day would imply it hadn't already been killed off, too. I'm still looking for the words to accurately pin down exactly what was going through my brain in reaction to this moron. I just... don't even... : /