Seems like to me the stereos keep getting louder and louder. They have setup laws here to help stop it, but they seem rarely enforced. How is it in your area? What do you think is too loud?
I've encountered many loud cars on the road. It only shows that the enforcers are not so strict about this matter. I always think that some drivers on the road are deaf or have hearing problems. I'm annoyed of loud cars so I always keep my volume on average level.
For the most part you won't hear to many people's stereos going down the road. I think the problem is more of the bass. That's what I can hear the most at least on most car stereos.
I think if the car beside you can already feel the pulse of the music or can hear your windows rattling or worse, can already identify the song..then it is too loud.
I will sometimes play my radio loudly, but only if it's a song I really like or I am in a really bad mood and need a pick me up. I do get annoyed with the super heavy bass, but I am a DJ's assistant so I am pretty used to it by now.
I always say if you can hear music coming from someone else's car then the stereo is too loud. Even though it maybe below the db threshold before human hearing becomes permanently damaged.