It has been answered I think a lot in this thread, but the game only allowed you to save at little sign's if I remember right, you'll find them at set locations through out the world inside and out.
I think it was a dumb Idea, that just complicated the game even more than it already was.
I assume the maker, Maker's were trying to stop save spamming, which some people proclaim are ultimate cheat's. (
I am not one of those)