Should I use earplugs in traffic?
Should I use earplugs in a bus?
Should I plug my ears when a plane flies above me?
Should I plug my ears when teachers voice sounds too loud?
It depends on many factors. Most cities have outdated buses that are too loud and too polluting. One time I measured the noise on a random bus in my city, the average level of noise on that particular bus was 78db(A). A level which I'm not comfortable with, so I wore hearing protection.
The bus I usually ride on to, and from, the train station is a newer bus with an average of 65db(A). I don't wear hearing protection on that particular bus. Unless it's friday night and there's a ton of drunk people on board, which can bring the noise level well over 80db(A).
When walking around in traffic you should take the same precautions. Evaluate the situation you're standing in. Are you walking/standing next to a super busy road with noisy trucks and busses driving by? Wear hearing protection... Are you walking/standing on a mostly empty road with only a few cars driving by once in a while? Don't wear hearing protection.
Same goes for planes... Evaluate the situation accordingly... They are too high up to cause any type of auditory damage so you shouldn't worry about them...
Human voices directed at you shouldn't cause auditory damage either, BUT you should take your precautions if you're sitting in a classroom with many students who tent to chatter loudly, or a busy restaurant with a alot of people. If you're among alot of people in a confined space the noise levels can get dangerously loud. I believe one study showed that noisy restaurants can bring noise levels above 90db(A).