I'm no expert (but I am an engineer) - but the idea of 'loud pipes save lives' seems a little like (simply) a justification for riding bikes with unmuffled exhausts, which the 'Joe in the street' can't (apparently) refute.
Dynamically speaking, the sound is behind (follows) the rider and bike - not in front (precedes) to act as a 'warning' and it's in the approach to other road users that most bikers meet their fate.....
Sorry, loud Harleys goes with the 'image' that HD want to promulgate - it's just marketing, that's all..........