I saw a sign in Scotland once which I didn't understand. It was a triangular sign with a picture of a horse and cart with a bloke sitting in a wheelchair on the cart. I assume it means "disabled drivers" but why a horse and cart? I've got a photo of it somewhere.
Perhaps it was a very specific sign warning of a riding centre for the disabled?
Something like 'no vehicles carrying explosives', hence it's most often seen in situations like tunnels. I've seen the same one in France.
Regarding the Men at Work sign - I'm the sort of person who takes delight in details. I love that sign, because if you look carefully at the chap's back leg, you can see that he's wearing wellies - there's a lip at the top of them.
I heard something on the radio about the intial design of them once, many of them were designed by a woman, and she recalled drawing the sign for cows in the road, and having to be corrected, having obviously drawn a bull...
She designed the Rail alphabet too.
http://en.wikipedia.org/wiki/Rail_Alphabet
I could get very annoraky about roadsigns. I've always liked the difference between the signs for 'accompanied horses'. Ours is very prim and proper lady on a horse at a walk, but I've seen some in France that are a chap on a cantering horse. It's the subtle differences that make you feel you're abroad, or home.