The street isn't the problem. Cars and trucks are. If those streets were filled with people walking, cycling, etc., the health penalty of living next to one would vanish overnight.
Why not? A lot of streets often seem busy simply due to the incredible space inefficiency of cars. What I like to do sometimes, when I wait at a traffic light with my bicycle, is to count the actual number of cars that cross during a phase. The amount feels large, but when you count, it's often not more than the amount of cyclists that cross in the next phase, and they take up just a tiny fraction of what cars need.
Sure it absolutely makes sense. But for me and my eyes 200 people passing by are somehow still less "busy" than 25 loud and stinky cars. That's what I mean with I need better wording
Generally, the busiest streets are pedestrianised; you just can't move that volume of people in cars.
(Actually, the _very_ busiest streets are probably major bus gates (where, again, there are no cars), those can have fairly ridiculous density at peak times.