The Hallway

I have a question...you know when you are walking down a hallway...usually it's a pretty long one, and you see someone coming your way...someone you don't know...or maybe someone that you kinda sorta might have seen before. When do you smile and say hi? Cause sometimes I find that I'm like way at the end of the hallway and so I wait until I am closer to do my smile and say hi, but then that person is not looking at me anymore. Is it because I waited too long? But then if I say hi and wave too early do I have to keep my smile and eye contact the whole entire way down the hallway, that's just awkward...idk!