Can we solve the problem of hallway etiquette?

This has driven me nuts for years. If you are walking down a very long hallway, and you see someone you know walking toward you from the other end, what do you do until you are close enough to wave and say hi?

Alternatively, suppose you are walking down a very short hallway in a place you frequent (neighborhood, work) and see someone you don’t know. Do you default to making eye-contact and saying hi? If not, why not? (There are several people who work in my building who avert their gaze and never say hi to anyone. This also drives me nuts.)