I'm not sure "large percentage" is a statement I'd agree with, my searching skills are failing me, do you have any kind of source for that? I'd be shocked if it was over 5%...
I live near a medium-busy street. I haven't seen actual numbers but it wouldn't surprise me if at peak hours there are over 100 cars passing per minute.
If 5% of those are overly loud, that's an average of a very loud noise every 3 seconds, and most of them will take somewhere between 5 and 10 seconds to come and finally go away. If you don't think that's large, we have very different noise thresholds.
I guess "large" is subjective. 1-5% is the ballpark I have in my head based on experience, which qualifies as "large" to me when I get passed by thousands of cars a day.
The hard numbers I'm aware of are about motorcycles, which have much higher rates of illegal modifications than other vehicles. This source documents a bunch of other sources, with estimates ranging from 40-70%:
