On my run Friday morning in Seattle-land I waved and said, "Good morning," to almost everyone I passed on the Sammamish River Trail. They mostly ignored me completely. Except the old people. Old people rock it. Jess and I are the oldest twentysomethings we know, and I frequently wish people got old faster. Anyway.
I grew up in the west suburbs of Chicago (Elmhurst, IL, to be specific). On the Prairie Path and Great Western Trail in that area, if you wave at people as you go by you have a decent chance to get a response. In Champaign-Urbana, where I went to college, there weren't so many opportunities -- on campus you're mostly dodging people and off campus you don't run into too many people on foot. When you do you typically give and get a wave. Cody is sort of like that, minus the campus part; when I pass someone on foot here there's almost always an acknowledgement.
Running on Chicago's Lakefront Trail is sort of like trying to run on the Quad at UIUC on a weekday in terms of how busy it is. Maybe it's more like a human-powered highway. I'd say its users act toward eachother basically like drivers on a very busy two-lane road. So it's not a great place to run. And Chicago's sidewalks, in many places, are packed with people trying to do very different things than you are. Nobody will wave to you in those places. But if you're on some of the minor trails, or in certain parks in the city, a few people will say, "Hi."
I would have expected, based on their purpose and density, that when running the trails at Rancho San Antonio in California, people would have returned my waves, like they do on the Prairie Path. But they almost never did. And I would have expected the same on the paths in suburban Seattle, but again, they almost never did. I don't think very many people consider Chicago to be an unusually friendly place (and many people consider Seattle to be one); maybe the Prairie Path is just an unusually friendly running trail.