What If a Traffic Sign Was Blocked, and Not Visible?
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 is not going to 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 it. Not noticing a sign is your mistake (See "/ Just Missed It" ), and you are required to follow all road signs.