Some people don't like the idea of God judging us, but are too scared to say so.
So a church billboard which says "You are under the judgement of God and are going to hell" would give them the screaming heebie jeebies, but they would have to find a way to say so without also attracting attention to the fact that they have stopped believing in the idea of divine judgement—if indeed they ever did.
Instead they will talk in terms of the need for respect, love, gentleness, humility and the desire not to be misunderstood. As if to speak truthfully about the reality of judgement, even if in a short-hand way, is somehow not respectful, loving, gentle or humble.
However, rather than giving slightly timorous people grief, it seems to me that we need a line of regular billboards which state gospel truths in a rather blunt and unvarnished way, and a whole 'nother line of passive aggressive billboards that don't really speak of judgement, but kind of hint that all is not well...
"We all share in the guilt"
"Why can't we all just be nice to each other?"
"God is very disappointed."
"I think you know what I'm talking about."
"Do I have to spell it out?"
"I hope you understand faint disapproval."
"Remember to breathe."
"Practice random acts of kindness."
"Watch this space"
Admittedly, some of these slogans are not particularly obvious. But their juxtaposition with a nearby church building should be enough to help people pick up the hint. I can almost now see the penny dropping—people reading such billboards, noticing the nearby church and falling to the ground convicted by guilt and crying "Woe is me! For I am lost; for I am a man of unclean lips, and I dwell in the midst of a people of unclean lips; for my eyes have seen the King, the LORD of hosts!" (Isaiah 6:5)