Just doing a random search today I came across this rather interesting commentary about a relatively new law implemented to regulate aftermarket exhaust systems:
http://www.sportcompactcarweb.com/archives/slipangle/slipangle_1201.jsp I must humbly admit I wasn't even aware of this law or that there were any preceding legal restrictions imposed by the state of California on noise levels of exhaust systems.
Honestly, this is yet another example of bureaucratic legislation in areas they have no business meddling.
Similar to noise restrictions on car audio systems, it all comes down to personal responsibility.
If the owners of vehicles who have aftermarket exhaust or audio systems would simply conduct themselves with some modicum of self-restraint & common sense, none of these intrusive legislative restrictions would be necessary.
Unfortunately, the ignorant few who feel compelled to "show off" their vehicles at inappropriate times & in equally inappropriate places thus causing public complaints only leads to government appartchiks & law enforcement officials becoming involved via legislation such as that imposed in California.
