Read this, it is very good
http://www.off-road.com/dodge/tech/Emissions_Legal.htm Here's what it boils down to:
"In a nutshell, there essentially are no legal exhaust modifications. According to EPA, pretty much any modification to your vehicle that in any way alters anything from the intake air to the tailpipe exit constitutes tampering. The only way an individual is absolutely assured of remaining in compliance after making modifications is to spend about $2,000 or more putting the modified vehicle through the full Federal Test Procedure (FTP) that the automakers use. Anything short of that, while not being an "enforcement priority", can be considered tampering. In short, unless you have the resources to put your finished project through Federal Test Procedures (FTP), you place yourself at the whim of a regulator's judgment. Should that regulator decide that your actions constitute tampering, you will be facing a $2,500 fine or the legal costs of attempting to fight it. If this comes as a surprise to you, you're probably not alone. Memorandum 1A states that the EPA will not consider any modification to a certified emissions control configuration to be a violation of the tampering prohibition if there is a reasonable basis for knowing that emissions are not adversely affected. In many cases, durability, aging and emissions testing according to the FTP would be necessary to make this determination. Whether or not your vehicle came with a catalytic converter stock makes no difference, the FTP for your vehicle are still in effect.
Is there good news? Yes. EPA doesn't intend to make this an enforcement priority. Essentially, that means that they maintain their legal right to fine you, but won't do so unless the political opportunity presents itself. "
Here's excerpts form the Montana vehicle code enforcement guidelines, if it's illegal in MT you can be positive it is at least at the same level, probably worse in most every other state. Heck, most of Montana doesn't even have zoning or building code laws, it's a lawless state.
"Excessive noise is primarily a nuisance issue rather than a safety concern, and determination of excessive noise is subjective. For this reason, enforcement personnel are to exercise sound professional judgment In making a determination of violation. The following guidelines and attached question and answer sheet (Attachment A) provide guidance to enforcement personnel regarding appropriate enforcement procedures.
Enforcement Guidelines
The only drivers who should be cited are those whose vehicles:
1) are not equipped with a muffler;
2) clearly emit an offensive, harsh, excessive noise, or,
3) have a clearly defective exhaust system (holes, leaks, etc. ).
Q1 Doesn't the VC (Vehicle Code) require a muffler on every vehicle?
Yes. Section 27150 VC requires that every motor vehicle subject to registration be equipped with an adequate muffler. There are no exceptions -- all vehicles, including trucks and motorcycles must be equipped with a muffler, as defined in Section 425 VC. A turbocharger is not considered a muffler.
Q2 Aren't all exhaust system modifications prohibited?
No. Section 27151 VC prohibits the modification of the exhaust system to amplify or increase the noise emitted by the vehicle, making the vehicle not in compliance with Section 27150 VC or exceeding the noise limits established in Sections 27201-27206 VC. Section 27151 VC does not prohibit all modifications to an exhaust system. It also does not prohibit all modifications that increase the noise level of the exhaust system over that of the original, factory-installed exhaust system (as it did until 1980). It only prohibits modifications that result in a noise level higher than those specified In Sections 27201-27206 VC. Accurately determining compliance with Sections 27201-27206 VC for enforcement purposes is generally impractical. Enforcement personnel must make an informed professional evaluation to determine if excessive noise statutes are being violated. "
I suggest you check your own state's VC to find out what applies, even if it's against the law whether it's enforced is a different story.
Attention: TDR Forum Junkies 
if i know right, your exhaust isn't suppose to be over 95 decibals loud. lately the cops in my area have been pulling over people (mostly kids) for loud exhausts. some are fighting the tickets, but it's just rediculous. it's usually the one of the bunch who's into him/herself who hates people/objects. there's been a lot of profiling in my town lately. 
heck, all the harleys around here have straight pipes, but now they're cracking down on them
Oo. )
