My understanding of the neuro-science (as of about 5 years ago) is that the RAS at the back of our brains is responsible for screening the 40,000+ variables per minute our brains "see" in our environment down to something that our conscious minds can handle and make decisions about. When it comes to driving, we are all trained a little bit differently. Most "I didn't see you" car-on-motorcycle accidents happen because the driver of the car has been "trained" to only see other cars and big trucks as threats worthy of triggering action. Our eyes are our dominant sense, and the "screening" dictates most of our reactions. When it comes to noise, the training our brains get is even more varied. Urban vs. rural, high vs. low background noise, etc. My buddy in the Air Force learned to sleep in his barracks under the take-off path at the end of an AFB runway, but it took time to train himself to ignore the noise. When audible car alarms first came out, all of us paid attention when they went off. Since most were "false" alarms, now we tend to ingore them and in some cases, not hear them. Some exhaust sounds are routinely ignored. Like some small engines, and higher-pitched exhausts (even on high-revving sport bikes). Some sounds aren't routinely ignored. Like an 18 wheeler starting to move. Like a loud Harley revving next to you. Like racking a round into the chamber of a 12-guage pump-action shotgun. The studies I've read say that when it comes to noise, our "ignore" vs. "take action" threshold varies tremendously based on our personal experiences. Taking a "loud noise" precaution to try to break through a car driver's RAS screening to get his/her attention may or may not work. But because we all screen noises differently (and because some cars screen outside noised much better than others), it may be worth something. It just can't be the only thing. Bright colors help, but drivers still don't see us. Loud noises MAY help, but drivers still don't hear us. Riding a motorcycle is inherently dangerous, and we all mitigate that danger in different ways. Defensive riding, constant awareness, good skills, practiced strategies for escape routes all play their part. Ultimately, we are each responsible for our own choices. Just make sure that you are looking and listening for the other guy, too.