Taylor is correct. ANY modification to the exhaust of a vehicle IS illegal. Enforcement of the law is left strictly up to the officers that patrol the streets. But do know that if you have any kind of aftermarket exhaust, you can be ticketed anytime you drive on a public street. Also, running without OEM catalytic convertors (ie; test pipes) can carry big fines, or worse. In fact, first offense is a second degree misdemeanor (up to 60 days in jail and/or $500 fine). Second or subsequent offenses are a first degree misdemeanor (up to 1 year in jail and/or $1000 fine in addition to revoking your license).
If you were to read into automotive laws, you would be amazed how many tickets you COULD get. Basically, anything than an officer of the law determines on the spot to be unsafe, you can be ticketed for. Riding too low? Possible ticket. Red or blue neons on the car? Possible ticket. Seat belt fastener held together with scotch tape? Possible ticket.