First let me say that I'm absolutely no expert on the topic. I'm just sharing my own experience.
I have a client in Zurich whose offices are directly on a very busy motorway. When the windows are open, the noise is deafening and you couldn't have a comfortable conversation in a meeting room. And the airplane noise from the nearby airport makes things even worse. When the windows are shut though, you hear absolutely nothing. It literally sounds like they achieve 100% noise insulation. It's incredible how well they work. It's a thick triple glazing (they look like three identical panes separated by a very small vacuum) within what looks like an aluminium frame.
I have never seen anything quite like it, including secondary glazing. When I moved into my current property, the windows were modern double glazed windows in PVC frames, but sound insulation wasn't great. So I added secondary glazing with stadip silence glass. It helped a fair bit, but still nowhere near those windows I saw in Switzerland. If I had to do it all over again, I'd go for some sort of triple glazing.