There is a general rule that drivers must see things that are obvious or clearly visible. For stuff that is blocked so that a driver can't see it, that driver will not be held liable for missing it. Some examples of this are if a traffic sign is blocked by a tree or by a big parked truck, the court will recognize that a driver couldn't have seen it.
While this might be helpful to argue if you are caught speeding, and there was a blocked speed limit sign, remember that speed limit signs are often posted along roads in several locations. Any visible speed limit signs you have passed, even if you didn't notice them, apply to you as if you had seen them. Not noticing a sign is your mistake, and you must follow all road signs.