He actually just has to yell/mentally assault you. Once you get within 10 tiles of his location, regardless of if you are through walls, or if you have the entire legions of hell between you and him, he can start to assault you with small amounts of the Ratting disease with every time he says something.
Also, my motorcycle in 0.C seems to work perfectly fine with just one wheel, what's up with that? Although I have to admit that imagining my survivor just wheelie-ing through the Cataclysm bring a smile to my face.
Probably because there's not that much difference between a motorcycle with a broken wheel, and an off-balance unicycle. One is a totally viable vehicle, and the other is absurd, but it's difficult to look at a random vehicle and say whether it is one or the other.