It's our culture to greet anyone who is next to you specially to older than you but now a days only very few people doing it.. However in rural area you have to greet anyone who you may cross over specially at night, if they will reply good morning too it means they are human but if they will not it means they are ghost.. I dont know if it a superstitious belief or its just a joke..

In Indonesia, we have to respect people who are older and our boss, but if you meet a stranger who is older than us and we do not know them, we are not obliged to give greetings

I have to say , if you are not familiar with some people , to be more polite is the best policy . Because people would judge you first sight by your manner or greeting . Never stress the importance of "HI" too much especially to those people who are not your acquaintance. I would rather too much greeting than being misunderstood .

Here we say "good morning/afternoon/night" to older people, bosses, neighbors, shopping workers or if another people say me "good.....".

Here in the UK, at least the part where I live, it's actually quite common to greet the elderly in the mornings if you're walking past them with a good morning, but more often that not it is the older citizen that says "good morning" first. However, I've never heard a good afternoon or goodnight said to them in the street... that would just be weird haha. The only other times that greetings are said to strangers in the street that I'm aware of is either Christmas or New Years Day with a "Merry Christmas" or "Happy New Year's" respectively :)