It is very common in US that if you see a person in the eye (if your looks connect with other person) when you are walking down the street or anywhere in a building, to say a friendly phrase,
“ Hi ! How are you doing ?”
Typically you smile and then say the phrase. This is a very common and most important way to greet people. Irrespective of if you know the person or not, you always ask the person “How are you doing ?”. It is a common etiquette in America. I was not used to greeting someone on the road or anywhere back in my country, but now I kind of got the feel of it. If you are a student, you greet the professor in same way, but add ‘Professor’ before .
If you do not say, “How are you doing or smile”, you just nod the head down just as a gesture to acknowledge the other person. You may say “Hi” and then nod the head down. I was not used to this too…In our culture, we do not say Hi to random people on the street and it is socially considered intruding someone’s personal space, but in US it is very common and it is the way you do it.