This is more problematic. Something ACTUAL has occurred, or has been noticed. The people must be warned. Which means only: we must instigate a Process of Warning. We must take all reasonable measures in an attempt to warn those who may benefit, from having been warned.
In any case of announcement or warning, whether governmental, or your library club dance social got canceled, or there is a werewolf loose. The Process of Warning is pretty similar. The process of warning is not that a person is warned, it is that a process is enacted to warn.
Step one: an announcement (could be a public announcement, could be a dedicated blast)
Step two: keep the announcement available, for those who missed or forgot about it. Keep it available until it no longer applies. No harm keeping it available after, if the issue it speaks to has possibility of recurrence.
Step three: referral. Anybody who missed it and comes to you, you've got a place to refer them to! "Snap"
Of course, "reasonable measures" is proportional. In cases of emergency, then we must bring in greater-than-reasonable measures. But we must always be realistic. With the exception of a direct 1-to-1 warning, you placing a hand on the shoulder, warning the person eye-to-eye, and than shooting them in the head (which will prevent them later claiming to have not received the warning, or not been warned, or not understood the warning, or not believed the basis of the warning to be credible), No possible Process of Warning will EVER SUCCEED IN WARNING ALL WHO NEEDED TO BE WARNED.
Partly, this is because communication is a fallible process.
Partly, it is because a lot of people are...let's face it. Some people need to be warned that the earth has an uneven surface, and presents a trip hazard. The appropriate response to these people is pity, mercy, charity. Do what we can to help them up.
However, towards the world and towards all those in it who are perhaps less pitiable than these, our responsibility is far stronger, far more important, and it is very clear: we must do whatever we can to keep the surface of the earth uneven. To keep the earth the glorious trip hazard that it actually is. To prevent and thwart those who believe it should be paved, flattened, made safe! Our responsibility extends even to flattening these people, if necessary, for their thinking is far more dangerous to people than any hazard they've supposedly noticed.
We must do our best to present the general warning that reality is real, and that it will therefore always present hazards. We must emphasize the general truth that each of us is responsible, and we're all going to die. We should advise others, "hey! Don't forget to look both ways! And don't forget, there are far more than both ways to look!" Danger lives not exclusively in the road. And then once we all feel adequately alert and responsibly warned: we should jump fences, and race furiously through the obstacles and empty lots that the world is covered with. Obstacles such as Everest, Kilimanjaro. Empty lots such as the Mohave. Let's race headlong, watch for furrows and take plenty of rests. Let's climb. Let's treat the city street like we would a treacherous cliff, for that is what it is: and far more treacherous, and we are not tamed animals.
We are civilized humans. And maybe civilization is a cage! But a civilized human is not an animal tamed by the cage its fellows have built and explained. A civilized human sees the use of the cage, and is fine with making such use. But a civilized human is wild, not tamed - with an enjoyment of the refinements culture can afford, but just as easily able to leave them behind and live rough sleeping on the steppes, or occasionally, on the steps. With a fierce love of the human dignity of others, a dignity which finds expression and affirmation in our social graces.
A civilized human is the animal that commands its cage. It understands its cage, knows the uses of its cage, and it knows where they keys are. They're right on the hook, hanging by the front door. Shall we go outside?
Let's. And when we do, let the earth be a trip hazard, rife with low walls, jutting edges and plenty of pitfalls. Well, all right. Let's learn parkour; because that's what it's for.
That's what earth is for.