I think everyone who's ever driven on an unrestricted stretch of Autobahn knows the experience of getting ready to pass a car in front of you: Very quickly, you learn to scan the rearview mirror searching for approaching vehicles in the passing lane before moving into that lane yourself. Ideally, you wait until you have good visibility of a long stretch of empty road behind you, to be sure you won't get flashed. Then, of course, as soon as you pull over and get about even with the car you're passing, a Porsche/BMW/MB/etc. appears right on your tail flashing its lights.

That's how you know the earth is round.