Here's my take. Cars are built to be soundproofed and quiet, to the extent it becomes a selling point. Seriously, some of them are like coffins - people get in them and hope to have as little external stimuli as possible. Add to that increasingly advanced bluetooth audio systems and satellite radio, so people are listening to music at full blast. How many times do you hear cars going down the street with their **** blaring for everyone to hear? You can feel them coming well before you even see them, because the reverberations transmit into the roads.
Even if you do have those loud pipes, a person not specifically listening for it may still end up tuning it out. People like us hear loud and quiet motorcycles alike - because we like them, and want to see other bikes. Contrary to popular excuse, people don't actually multitask well - the brain will ignore certain stimuli in favor of others. They may "hear" it but not actually listen to and acknowledge its significance. Of course, the moment they get a text message, they'll hear the ding over their gangsta rap and be Johnny on the spot in answering while at the wheel.
The same thing happens with visual stimulus - even when I dress like a clown with my high viz jacket and bright white helmet and high beams on in the day, on a white bike, I've still had people turn in front of me, and tell me they didn't see me. Of course their eyes saw me, they just weren't processing, and the brain just threw it out as insignificant, because they were looking for cars. Or who knows, maybe they weren't, some people are just born stupid, and don't have to work for it.
So, I don't think loud pipes necessarily save lives any more than high viz gear. It might snap a person who's only somewhat distracted out of a trance, but then again, so will pressing a horn - a sound people are a lot more accustomed and likely to listen to, I might add. Maybe there is benefit, but also think of the costs. High viz gear has no real downsides, as far as I know, but loud pipes could damage your own hearing, get you pulled over if it's illegal, and piss off your neighbors. Though the latter might be seen as a pro!
Some cars have technology like blind spot warnings, and if I remember correctly, some luxury cars even have tactile feedback - you'll feel a vibration in the wheel if you attempt a lane change with an obstruction. We have radar guided cruise control that works in stop and go traffic. All of this technology, designed to aid humans in conducting a task that requires inhuman amounts of concentration and attention to detail. Still, life always finds a way, just like stupid people always find a way. Ride like you're invisible, and ride like no one can hear you. For the latter: many do, it's a well known fact that those who rev the **** out of their engines at intersections believe they pull unimaginable amounts of tail. They really don't, but who am I to judge?
