The distance of your radius that you will acknowledge the people you know and greet them. Greeting radii almost always depend on person. Usually, your greeting radius varies based on how much you like the person. Someone you are close friends with, will probably have a greeting radius of over 50ft. For people you don't like, it would probably be something like 10ft if it's crowded where you are.
Steve: Oh dude, I had to say hi to Kent today?
Jon: Aww, you said hi to Kunt? He probably thinks you like him now.
Steve: I know, but I kinda used to know him, plus, he got in my greeting radius
Jon: Shit, sucks for you, haha
Steve: Yeah, GOD today was a bad day