Motorcycle exhaust volume is regulated at the federal level, and may also be regulated by individual states. Open pipes (also known as loud pipes) are in fact illegal to use on public roads as regulations at both the federal and state level restrict maximum decibel level for on-highway use.
Federal law sets volume restrictions based upon a motorcycle's year of manufacture. A motorcycle manufactured through the year 1983 may not exceed 83 decibels. Motorcycles manufactured from 1986 must be quieter, as the bike may not exceed 80 decibels. Federal law prohibits tampering with the exhaust system of a motorcycle.
Several states also provide for restrictions on a motorcycle's exhaust levels. For example, the Illinois Section 902-101 requires a muffler to remain on a motorcycle, and any muffler on the bike may not emit noise levels greater than the factory muffler. Section 902.122 restricts maximum decibels to 80 decibels at speeds of 35 miles per hour or less and 86 decibels for speeds greater than 35 miles per hour.
On-highway refers to use of a motorcycle on public roads. Where the motorcycle is used for off-road use on non-public or private roads, exhaust decibel restrictions do not apply.