If you think its going to be a good photo take it and explain yourself after. Most people are fine with it so they will just continue doing what they are doing. I'd find it awkward and a bit annoying to talk to every person I photograph on the street, but happy to speak to anyone who wants to ask.
Check laws about public photography but most places its fine to take photos of whatever you can see.
You're limiting yourself. For me street photographers are capturing life so everyone can look back in time later. It's not about the person in the photo, it's about how people live today and capturing it.
I haven't had any issues and in my experience most people don't mind or even notice, some ask for a street portrait because photographers take good photos.