To be honest, it was a bit of a sport for me at one point.
I was part of a group of about 10 people around Adelaide and we were always just trying to get signs that were "cooler" than the last... And the 50 was just cos I think 50kms on the road that its on is frickin' ridiculous! lol
And I understand its a safety issue, thats why i never took any signs that
a) there wasnt another of (ie. the black arrow being on a corner with 5 other black arrows, and the danger high voltage sign was placed about 3 meters from another one on the other side of a doorway into a powerstation)
b) were stop signs, give way signs, intersection markings or anything like that
c) didnt have some kind of reason. Meaning, i didnt ever take a sign for no reason. It was either a challenge or i specifically wanted a sign of that design for a particular reason.
Anyways, im not going to defend my point any further. In theory it may seem like some of them could be missed (meaning, without them it poses a threat) but if that were the case, I wouldnt have taken them.