You can actually be fined for having car music too loud, it has happened.

A large part of the problem is the arms race in amplifier sizes. There's hearing clearly at close quarters and being able to hear clearly two hundred yards away. There's an act I can think of that uses a generator driven PA and another that uses two 500 watt powered speakers. A full drum kit can be heard at a quarter of a mile! Such buskers effectively kill a wide area of a town centre for other, quieter buskers.

How far away is it reasonable for an act to be heard clearly?