Why are motorcycles permitted to be so loud and cars need to be so quiet?

The only 'permitted' in loud motorcycles is a reluctance of law enforcement to prosecute the violators that willfully alter manufactured motorcycles that must meet government regulations before they are sold. 

The myth that 'loud pipes save lives' has no statistical support... NONE!

Motorcycles have the same muffler laws as cars do.    People illegally replace their mufflers with illegal modified mufflers.    It is just a way to show off.  They have no respect for other people's peace and quiet.

It is true, being noisier may help you not get hit by inattentive drivers. You never rode a motorcycle and have no clue. They law didn't change for motorcycles, but sometimes it isn't worth the trouble to hassle them for a little more noise. 

(Best answer, adam.)

 All vehicles sold new are required to pass  Fed dB max stds, Stamped Right On the mufflers of a MC. Even Sez: "illegal to modify so as to exceed". Must pass   state/local regs, too.

Its just that bikes are not pulled over as much as cars.Troopers and cops Know they will only kill themselves.

If you hear some MC you "think" is "too loud", it had baffles punched out. Even aftermarkets must pass. Cagers do this, Also. I'd rather hear an opened up Harley than a Civic--Nyahhhhh..

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My V & H's are wide open; I Bought her that way. Yes, sound Does go Forward, but not as much. But, with the cagers here and their AC up all the way. windows up, , stereo Up, and texting, most  Still  don't notice. So this loud pipes saves lives works, Sometimes. If it  were 100%,  then Scooter riders would knock their baffles out, too. 

"loud" is a Ma Deuce. Or ANFO. Or an F-16 right over your head---water in a glass wobbles, the tarmac shakes. Never heard one break Mach One but I hear it can shatter windows. 

"Shorter Exhaust"? BS. I've heard new Harleys barely mutter. A 250 c Honda sounded like an Electric Motor. Exhausts can be Tuned to be quieter,  no matter how short they are, and crossovers help.  You can tune a firearm's  suppressor to a specific load, quiet But shorter, too.

Why are wannabe-ricer race cars allowed to be so loud? Those fart-can equipped hornets are more of nuisance than motorcycles where I live.

Falsely generalize much?

the LE Veloocett was so quiet it would be next to you and you would Not hear it

MY BMW R65 is very Quiet and Harley Davidson many say they have Patented the Sound

Bikes are louder because they have a relativity short exhaust system. It's not practicable to fit in large or multiple mufflers as it is in a car.

I've heard the myth 'motorcycles permitted to be so loud and cars need to be so quiet' but I've never found that to be true.

Because motorcycles can easily be missed especially if you're driving a lorry or other large vehicle.