I hope somebody will clue us in on the proper protocol. It always makes me nervous. Especially when I'm coming up on somebody from a minority group because I'm afraid that my social ineptitude will make them think that I am a racist... anybody ever feel like that?
This was one good thing about living in Central America. There, if you pass somebody on the street, you will always smile and give them the polite greeting most appropriate for the time of day. Excepting in large crowds, of course, in which there is no social obligation to meet the gaze of others or speak to them.
When I first came back to the states, it really bothered me how we all ignore each other on the sidewalk and I couldn't even remember how that ever felt natural. So I would smile at people and say, "Good afternoon!" (or whatever) This unfortunately led to several incidents in which people either thought I was bizarre or thought I was inviting romantic attention. So I'm back to ignoring people on the sidewalk unless spoken to... except I greet the elderly, who seem to welcome a smile and nod.