I live in a country where pretending your fellow beings don't exist is the pinnacle of polite behaviour. You don't say hi to people on the street unless you're drunk, have ingested something illegal, or are slightly mad. So if somebody tried to say hi on the road to me while I was biking I'd probably be too busy wondering why and whether I should be worried about it to respond...

I'm being a bit facetious here, but not only. I would try to respond I guess, but I'd be mighty surprised and might just stare. But it sure wouldn't be because I thought the person looked like a Fred or a dork or whatever, when I pass people I'm usually deep inside my own brain and the only thing I'm thinking about is "which is the safest side to pass on?" I guess it is a training thing.

FWIW I'm having a blast, even though I'm not grinning. And sometimes I am