Up ahead I notice there's three cars in a horizontal line, traveling the speed limit. Which is, to put it mildly, odd. Left lane to right lane often has a speed difference of 20mph or more - horizontal lines just plain don't *happen*. To say nothing about that many cars traveling the speed limit, unless they can't drive, and then they always hide out in the right lane. Nevertheless, they've left one lane to their left open, and I'm about to take it when I realize what's going on.
The rightmost car is a cop car.
Time to slow down!
So I join the formation, staying in my lane, continuing along at precisely 3mph above the speed limit, exactly matching the cop car and the other cars.
After a while, another car comes up behind me, doing perhaps 15 above, and makes obviously the same set of calculations and slows down as well.
This happens repeatedly until we've got about a dozen cars, carefully going the exact same speed as the cop car.
At this point, as near as I can tell, the cop says to himself "Screw this, this is dumb" and slows down 10mph below the speed limit so we can pass him without breaking the law. Which we do . . . and then, a few minutes further on down, all accelerate back up to 10mph above or so and spread out. (With a few guys moving ahead at 15mph above, and a few dropping back at 5mph above. As usual.)
Laws are a funny thing, y'know?