If you make eye contact and both of you are recognizeable then give a knowing nod or raise of the eyebrows as soon as. Then when you are a meter or two apart (just before reaching distance in order to shake hands etc.) greet then then make tactile contact.

I cannot give you one-set answer, I cannot even give you one-set of answers, for all situations. Since almost every culture (in the world) has a different distance, called personal space, and I cannot remember what the distances are. If you are in a business setting and success depends on it, you could to research the person's or people's culture and than adjust the personal space you give them by the research results. In some cultules (China and Japan and etc) business deals have been slowed by this problem. However, in causal greetings it probably is best to read the person's reaction, to the personal space you are used to giving them. If that does not work and you run into them again, them just ask as they approach.

I usually wait until I am within ear shot of being able to greet them and hear a response.

Wow this was a problem i had way too often. you are both walking down the hall and you recognise someone at the other end. this is followed by a series of awkward smiles and glances away and back at them again, untill you are within earshot and both let out an awkward "hi". avoid this painful procedure. i have taken to shouting down the corridor "cooey!! hello there, HOW ARE YOU?" then in my anticipation of the reply i skip towards them and kiss them on the cheek. This ensures there is no awkwardness. And incase you're wondering, yes it applies to everyone i meet in a hall, regardless of whether i know them or not.

Whenever I'm asked difficult questions of delicate etiquette like this one, I invariably turn to my dog-eared copy of The Meaning of Liff, that indispensable companion to good graces: CORRIEARKLET (n.) The moment at which two people approaching from opposite ends of a long passageway, recognise each other and immediately pretend they haven't. This is to avoid the ghastly embarrassment of having to continue recognising each other the whole length of the corridor. CORRIEDOO (n.) The crucial moment of false recognition in a long passageway encounter. Though both people are perfectly well aware that the other is approaching, they must eventually pretend sudden recognition. They now look up with a glassy smile, as if having spotted each other for the first time, (and are particularly delighted to have done so) shouting out 'Haaaaaallllloooo!' as if to say 'Good grief!! You!! Here!! Of all people! Well I never. Coo. Stap me vitals, etc.' CORRIEVORRIE (n.) Corridor etiquette demands that once a corriedoo (q.v.) has been declared, corrievorrie must be employed. Both protagonists must now embellish their approach with an embarrassing combination of waving, grinning, making idiot faces, doing pirate impressions, and waggling the head from side to side while holding the other person's eyes as the smile drips off their face, until with great relief, they pass each other. CORRIECRAVIE (n.) To avert the horrors of corrievorrie (q.v.) corriecravie is usually employed. This is the cowardly but highly skilled process by which both protagonists continue to approach while keeping up the pretence that they haven't noticed each other - by staring furiously at their feet, grimacing into a notebook, or studying the walls closely as if in a mood of deep irritation. CORRIEMOILLIE (n.) The dreadful sinking sensation in a long passageway encounter when both protagonists immediately realise they have plumped for the corriedoo (q.v.) much too early as they are still a good thirty yards apart. They were embarrassed by the pretence of corriecravie (q.v.) and decided to make use of the corriedoo because they felt silly. This was a mistake as corrievorrie (q.v.) will make them seem far sillier. CORRIEMUCHLOCH (n.) Word describing the kind of person who can make a complete mess of a simple job like walking down a corridor. The Meaning of Liff Douglas Adams John Lloyd