New England guy here. I was surprised when I read OrphanWilde's comment yesterday; I went out last night and observed. These are the rules most of us follow:

1. Someone trying to initiate eye contact wants to talk to you -- even just "hey, how are ya" or a gesture of acknowledgement like when you pass them on the street. But the eye contact is always a prelude.
2. If you don't want to interact, look in their general direction, but not into their eyes. If you do catch their eye, either look away fast or give 'em a nod or make your eyes wide or something.
3. You get one freebie look when you walk into a room or get on the bus, or when someone pulls up next to you in their car, that sort of thing. Gotta know who's there.

It feels kind of evasive, I guess, but I don't believe that in other parts of the world the rules are much different than this. Especially other cities, obeying the meta-rule of "your acknowledgement of a stranger is inversely proportional to the number of other people around."

I don't see any connection to aggression, creepiness, or man/woman.

Some examples:

• I'm at the store trying to find something. Someone walks up beside me to get something. I ignore them, grab the thing, leave. Eye contact in this situation would be unusual.
• Walking past someone on the street. Here eye contact is optional, but mostly avoided. In Boston you generally will not get a response from a greeting anyway, but in Western Mass you will. In Boston people won't even notice you're trying to look at them, most of the time, so in those cases a verbal greeting is actually surprising.
• I sit down across from you on the train. Take your freebie look if you want it, greeting optional. For the rest of the ride, don't look into my eyes. The first time you do it, I'll give you the look-back. The second time, I'll start a conversation.
• Airplane. Completely average to never look into the eyes of your neighbor. But those have people from all over on them.