'Expect the unexpected'

On my drive to work this morning, the digital sign on Pont Champlain said 'Expect the unexpected. Drive carefully'. What the hell does that mean? I'm all for illogical sentences ('where did you lose it?'), but on a street sign I expect a little more. It's the equivalent of those road signs warning you that deer might cross at some point, or rocks might fall on you - how am I supposed to process that in a useful manner?

Expect the unexpected and drive accordingly - if we really followed that advice then we'd all be doing 5mph and have mattresses strapped to all available surfaces of the car. And be wearing a helmet. Scratch that, we'd probably just stay home with the tinfoil hats on, ordering out for food that we wouldn't dare open the door for.

Why isn't 'don't drive like a dick' sufficient? Everyone would get that and behave themselves - why the need for nannying statements that make you want to do the unexpected just to fuck with everyone else. 'Hah - I did a handbrake turn while pulling a mooney out the window. You crashed into the barriers because you weren't expecting that'. I'm not sure it would hold up in court, but still....