Someone is serving on a famously foggy stretch of highway, which has recently been equipped with a new accident-prevention system.
"Hm, I wonder what they think will work here."
After a while he sees a series of signs, first a few with writing on them, then widely-spaced circles that seem to go along for the whole stretch:
1. If you can see one circle, the speed limit is 50.
2. If you can see two circles, the speed limit is 75.
3. If you can see three circles, the speed limit is 100.
4. If you can see four circles, you just rear-ended an Audi.