I've never seen the right answer for this recorded anywhere. It's one of those things that everyone more-or-less plays by ear.
I think that many of us–myself included–stumble through the awkwardness of passing strangers on the street with a mixture of attempted poise, acute self-awareness, trepidation and frustration for the whole thing. And this is just the self-directed stuff. I also think that if we had our way, none of the circumstances that caused us to become hesitant to look someone in the eye would have happened in the first place. We want to be honest and open but we don't know if the other person is capable of holding our gaze, however fleeting, and returning a commensurately genuine response. Do I nod up? Or down? Do I look twice? Again. Did they even see me? If so, do they think I'm ignoring them? I'm not. I don't want to appear rude. I know I'm not rude, but do they? Do they even care. Do I? Oh well.
It takes less time to mentally juggle this cascade of questions than it did for you to read it.
We think at lightning speed. But the Holy Spirit is quicker. I know that honesty wins the day and that if we're honest with Him, He'll make sure any interaction goes off correctly and without a hitch.
This is the conclusion I've come to. If you know a better one, let me know!