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For years now, six days a week, my doggie Einstein and I take a 45-minute hike around a lake that is so popular that each time, I pass 100 people walking in the opposite direction.
During those walks, I've made an informal study of people's eye contact as they approach me and others walking in my direction.
As a result, I have identified five types of eye contact and have developed hypotheses as to what they mean.
Looking at the person's face for longer than one second. This is the rarest type of eye contact. Most often, the recipient is about the same age as the looker and is conventionally attractive.
I observed that, on average, men are more likely to look that long at a woman than vice-versa. When a woman is doing the looking, it's more often at an attractive woman. One hypothesis is that's even in today's feminist era, many women feel it's too aggressive to look that long at an attractive man.
When seeing a sexually attractive person, many Americans of both sexes try to maintain a deadpan appearance but it's hard to control such things as widening of the eyes or a slight upward curl of their mouth's corners.
Of course, there are false positives, which can occur, for example, if the person thinks s/he knows you or likes something on your face: eyeglasses, earrings, etc.
Looking at the person for 1/2 to 1 second. This may suggest interest, perhaps sexual attraction, platonic attraction, or simply that your appearance is interesting enough to warrant a glance, for example, if s/he likes your hairstyle.
Looking straight ahead. The person can still see you with peripheral vision but has looked directly at or away from you. This can mean one of many things: S/he is spacing out, shy, introverted, thinking about something else, or neutral to your appearance. The latter often is the case when a young person sees someone who is much older. Looking straight ahead is more likely to be a reaction to you if, 10 to 40 yards away, you notice him or her looking at you and as s/he gets closer, stares straight ahead.
Looking away without changing facial expression. That may mean s/he finds you moderately unattractive and/or that s/he's responding negatively to your having established eye contact---It says, "I'm not interested."
Looking away with a look of disdain. After a quick glance at you, the person looks away while his/her face goes from flat to scoffing, for example, upper lip curled in disdain, flared nostrils, or even a sneer that says "You gross me out." Of course, it could also be that, at the moment, s/he had a negative thought having nothing to do with you.
A moment of eye contact can send a signal, so might you want to be more conscious about how you look at people?
Marty Nemko's bio is in Wikipedia. His new book, his 8th, is The Best of Marty Nemko.
