Until this question was raised as a forum topic, i never thought about it till now. When i am out running, which is always on my own, and with my MP3 player on, i do a quick nod to some people, i suppose all people, but it might be so subtle that a lot of people might not even see it. I believe in running manners when in a race, but during the training time, i do sometimes find that i am so focussed on my breathing, my pace etc, so dont worry too much about what is going on around me. If i nod, and the other person doesnt nod back, i dont think bad of that person, since i always feel, that i dont know that person, what is going on in their lives, and hence cannot judge them for being rude. I sometimes do run out my bad day, and one time i even ran past a work colleague without knowing, until the next day when they told me.
I think a lot of people do go into the zone. Its not being rude, but it can come across that way, but at the end of the day, how do we really know what that person is feeling.