signs just make people feel better. a guy that will trespass without a sign will trespass with a sign. the exception might be the border with public ground.
In my state and many others, it's not illegal unless there's a sign. And some people do pay attention.
not quite true.
In Virginia - you can post it with a sign, or with a silver or purple paint.
it is illegal to hunt posted property without written permission
it is illegal to hunt unposted property without verbal permission