I make a practice of saying 'hello' to people I pass on the sidewalk, weather I'm running or biking. In my experience, I'm occasionally surprised when I'm preempted by someone else who says hello first. I've noticed that the frequency of this occurance seems to be correlated to how closely people live to one another.
I should admit that my esperience is limited to various degrees of city and suburb. Perhaps there's more of an inverted bell curve of friendliness from rural through suburb to the city, but I have no experience with the first half of the curve.