Per this study, it's just not effective.
Actually, Loud Pipes Don’t Save Lives
Despite what the bumper sticker proclaims, it’s totally not true.www.autoweek.com
Do Loud Pipes Really Save Lives? NoResearchers found that even the loudest pipes are very hard to hear in a modern car.At 50 feet behind the car, “none of the motorcycles in the test can be heard inside the car.” At 33 feet behind the car, “even the noisiest motorcycles tested can hardly be heard inside the car.” With the motorcycles’ front wheel next to the car’s rear wheel, one of the motorcycles can be heard inside the car and three motorcycles can almost be heard but, “unfortunately it is too late to be safe.” With the bikes 13 feet in front of the car the motorcycles cannot be heard. “Car sound isolation in front is better than sound isolation from the side.”The conclusion of the study specifically says that the assumption “loud pipes save lives” is false. The driver of the car cannot hear a motorcycle if it is more than 33 feet behind the car, and as it gets closer than 33 feet to the car “even if the car driver hears you, it is too late to react safely. So we consider that noise is not a warning for the car driver. It can even be considered a danger because you will not have time to adapt to the new reaction of the driver.”
Flawed study. Motorcycles behind cars are not the riskiest scenario. Having the two vehicles at right angles (like a car pulling away from an intersection with a bike going through) would be more meaningful.