Here is a post from a Triumph forum. I know that opinions vary on this tribe as to whether motorists can't hear pipes, or if the noise just pisses them off. I found this interesting. Not exactly scientific... but one of those things that seems to have merit. Just as I have noticed that when cages turn in front of me, my high beam is always off.

"I stumbled onto an interesting social experiment once with regards to
pipe volume...

I worked at the same place for almost 10 years and lived at the same
house for 5 of those years so I took pretty much the same route to work
every day for those 5 years.

At the time I had a VTR1000F and had drilled out the baffles so it had a
nice (not offensively) loud note. Then I wrote it off (another story)
and decided to replace it with another VTR1000F but while I was running
the second one in, I left it entirely stock so I didn't change the
exhaust at all.

The difference in "safety" was amazing!! I rode the same route, at the
same time, with the same gear, same colour bike and with the same riding
attitude/practices - but immediately I had people trying to kill me!!
Just the normal stuff - changing lanes over the top of me, etc, etc,
nothing malicious, but very, very obviously more frequent. I had one
memorable 500m stretch where 6(!!!!) cars tried to merge on me one after
the other! I would have thrown my hands up in the air except I was
looking out for #7...

Of course, at first I thought I was just having a string of bad luck but
then when I finally drilled out the cans again, it all stopped.

So, while I'll be the first to admit that I really, really like a
louder-than-stock exhaust note simply because of the noise, I also stand
by the fact that there is a level of safety added by giving other road
users an "auditory warning" of your approach.

Noisy Pete (as my 2 y/o niece calls me)

ps note the correct spelling of "colour"..."