Hate to be that guy, but I think the sign is actually meant for the daughter, not the father. The rhetoric isn't "hey, maybe don't have sex with your kid?" it's "yo girlie, those things your father did to you were totally not okay, even though he was drunk/apologized afterwards/whatever."
I get that that still sounds painfully obvious, but trust me to people for whom "abuse" is their "normal," messages like this handing out really obvious messages are still useful
It probably helped prevent a single rape, but that was because they live directly in front of the sign and saw it illuminated by lightning a day they were drunk.
Coincidentally, the exact same number of people who have been convinced to stop gambling by a giant casino billboard ad with a tiny 800 number at the bottom.