I was just thinking about this today. My proposal was to replace them with behavioral nudges[0]. Things like "Traveling the speed limit saves X lives and gets you home faster[1]." Basically, they'd be gentle ways of making people more conscious and aware of the road and themselves.
But of course, who gets to pick what the billboards say? And I'm sure nobody would go for it if it were slogans from the State Dept. of Nudges...
Sounds like the BRO (Border Roads Organisation) signs you can see in the Indian Himalayas. These are some of the scariest (and most dangerous) roads in the world, so it's a special sort of relief whenever you come across their subtle life-saving jokes:
(My assumption being that English is a 2nd language for you...) If you're referring to the "Mr. Late" and "late Mr.", it's a play on words. "Mr." being a male title in English, and the dead are sometimes referred to as "the late Mr. Smith" (why is a lifelong mystery to me). So it's better to be the guy that's late ("Mr. Late") rather than speed down a dangerous road and be dead ("the late Mr. Smith").
The humorous "be careful or you could die" poems are also a feature of some bike events I've done, like the Marin Century. They particularly urge people not to descend mountainous curves faster than they can keep control. Which I guess is exactly what BRO is doing with its signs.
Anti-speeding PSA billboards are actually a thing on motorways in most(?) of Europe. Not so much in cities but most people speeding in cities are either driving at much lower speeds or criminally negligent (e.g. illegal street racing) in the first place.
But of course, who gets to pick what the billboards say? And I'm sure nobody would go for it if it were slogans from the State Dept. of Nudges...
[0] https://en.wikipedia.org/wiki/Nudge_(book)
[1] Real data of course. Citations needed.