You could say it was supposed to be there.

Before I read this story, I had always thought that that sign was put up by Caltrans as a semi-permanent thing until they replaced the aging sign altogether. I've driven all over the place in LA for it not to matter because I knew of the (very badly designed and unlabeled until the very end up until about a year ago) exit to the 5 freeway, but it was probably a fantastic thing to people new to the area, since that general location is where something like 3 freeways meet, and if you wanted to get on the 5, there was a big sign for 5 South, but nothing whatsoever for 5 North, unless you maybe saw a small sign or something (I forget what it was) that said to take the 110 North to get on the 5.

Do you think there's some government official that drives from sign to sign, comparing each to his notes on what it should look like?

That was the point of the original artists 'art' - he made the sign old and dirty, so people would assume it had always been there and they had never noticed. It was about what people notice in their surroundings.

If you passed an old broken down house on a lot you had never noticed before, would you assume you had somehow missed it - or that somebody had created a mock broken down house?

It doesn't help that in LA the signs are put up by different state, city, county and municipal authorities that don't seem to ever work together.