It doesn't need to work, it needs to be a clearly demarcated legal boundary. If it's hard enough that it takes effort to cross, you can prosecute. Someone who

It doesn't need to work, it needs to be a clearly demarcated legal boundary. If it's hard enough that it takes effort to cross, you can prosecute. Someone who wanders in the woods might not be blamed for trespassing. But someone who hops a fence with a sign on it doesn't have much defense.