The majority of motorcycle related fatalities are caused by vehicle drivers not seeing motorcyclists and turning in front of or into their paths. Bikers have a saying, “Loud Pipes Save Lives”, and that is a fact.
If a newer motorcycle is running its headlight is on. That is for greater visibility. But vehicle drivers often do not see the light as it may be in their blind spot and that is where the loud pipes come in.
Far too many times while riding I have seen a driver just ahead of me look into his or her mirror, not see me and then start drifting into my lane. A loud rapping of my pipes and their actions were aborted. They didn't see me, they heard me, and a collision was avoided. That’s not to say I deliberately ride in their blind spots, I know better. That’s not to say I make excessive noise with my pipes either, I know better. But as a biker you sometimes find yourself in a driver’s blind spot. And these incidents of drivers coming into your lane can happen before any adjustments to your position can be made. Loud pipes aren't just to be macho and have a loud bike, though I have no doubts some riders do so for that very reason.
There are no studies that I am aware of that confirm loud pipes do indeed save lives. But I have aftermarket pipes, and I believe the vast majority of bikers who also have them have them because we know based on our own experiences that at a minimum, loud pipes do prevent collisions which in turn saves lives.