I tend to be a more reactionary person when running. I always look at the approaching runner, and if they make eye contact - I smile and nod. If they look blankly ahead, I do the same. Fortunately, I wear sunglasses almost 100% of the time when running, so I don't look too
I'm pretty much a 100% waver / nodder when cycling. Totally not dependent on the other person's reaction to me at all. If you're on a bike and pass me, you get a wave...or a nod. And smile if possible. I can be climbing a hill and be so far down in the pain cave that I can barely breathe, but I'll STILL, at a minimum, do the pinky flip to a passing rider as a form of acknowledgement.
Responses When Running:
Here's what's interesting (to me anyway, you're probably wondering how you can get the last three minutes of your life back) - the percentage of people who make eye contact has been directly dependent on where I'm running. You see, I primarily run in two areas of town - East Nashville and Brentwood. When I run in East Nashville, almost everyone makes eye contact with me. When I run in Brentwood - not so much. I'd give it about a 25% acknowledgement level. Now, this could be because my running club, East Nasty, is based in EN and there is a decent chance the passing runner and I might know each other. Or it could be that Brentwood is just a bunch of snobs (though EN has hipsters and isn't that really the same thing?)
Responses when Cycling:
I ride all over Tennessee, so I can't base others reactions by location when riding. But overall, cyclists wave the majority of the time. It's good to see someone out on some lonely highway while you're out riding. But truthfully, waving is also a good way to check out each other's bikes. And we do.
Here's a poll: What kind of athlete are you? Are you a waver? A nodder? And if you are - do you get miffed when you acknowledge someone passing and they ignore you? Or are you a strict non-waver? Let me know!