Is it actually true though? Compare these two comments buried downthread:
https://news.ycombinator.com/item?id=16470358
> The hitchhiker's paradox is correct, taking a point and looking backward or forward will correctly give an average event 10 minutes away, but combining the events to give an average of 20 minutes is false.
https://news.ycombinator.com/item?id=16471544
> I thought this sounded funny, and I did a little simulation to see if it was correct. Given his assumptions (poisson with lambda 10), you do not get that answer. I got right around 10, which is what I would expect.