The sign at right (or something similar) is relatively common in certain areas of Vermont: Moose Next 5000 FT 40 MPH.
Ages ago, the first time I saw such a sign, my first thought was, "How do the people who put it there know where the moose (mooses?) cross?" I mean, it's not as if we're dealing with a herd migrating on a predictable schedule on an easily observable route. We're talking about moose (meese?) here, not the swallows of San Juan Capistrano. A moose is liable to cross the road at any time, just because, and with potentially disatrous results.
The answer is not so much when, but where. Moose do follow (after a fashion) the same rule as water, electricity, and other natural occurences: a concept known as the path of least resistance. In their need to go from a certain place to another certain place, they will follow a relatively predictable route because it is the most direct and easiest. So, over a period of time, the people who decide on where to put the signs will acquire a reasonable idea of where moose, deer, etc. are likely to cross the road.
Now that the natural history lesson is over, let's address how the sign says what it says.
If you encountered such a sign along the highway (this particular one is along Route 105 in Brighton), would you know when you had actually passed through the warning zone? Would you know how to determine when you had actually gone 5,000 feet? Speed is irrelevant; this is distance.
One of the myriad facts I have at my immediate disposal is that a mile is 5,280 feet. Those bad-tempered old women dressed like penguins (a/k/a Dominican nuns) at St. Mary's Elementary School (Poughkeepsie, NY) had a way of indelibly marking such tidbits on your brain, things one needed to know in order to solve those homework questions that started out "John leaves home traveling east at 40 miles per hour," yaddayaddayadda. Then again, maybe it's just me: I still remember the address and phone number of my early childhood home in Washington, DC. I was six when we left there in 1955.
At any rate, because I know the length of a mile in feet, I know that for each tenth of a mile, I have gone 528 feet. Thus, when the last digit on my odometer has flipped nine and a half times, I have gone roughly 5,000 feet (5,016 to be exact, but who's quibbling?). I'm not saying I actually do this, just that I can if I wanna. It's much more important to watch for moose.
How many people have the factual knowledge upon which to base the calculation, so that they, too, can if they wanna? My next door neighbor was vaguely aware of it, which seems to be the most common answer, just among the few people with whom the subject has come up in casual conversation.
The way this sign is worded requires some thought and concentration, if it is to be of any use. The honest-to-goodness bottom line is, a) why take a chance that a given driver knows how to perform the calculation, and 2) do they really want people watching their odometers instead of looking for moose? Why can't the sign just say "Moose Next Mile" and get it over with? We get a lot of Francophones from Quebec here. In general, is their English good enough to realize the danger? In this age of icons, should there be one sign with antlers and another with a red diagonal in a circle over the antlers? I move that highway signs be required to adhere to the concept of KISS.