I don't know where you live......but I found this on another forum,
In Washington they basing it off this RCW: 46.37.530
(3) For purposes of this section, "motorcycle helmet" means a protective covering for the head consisting of a hard outer shell, padding adjacent to and inside the outer shell, and a neck or chin strap type retention system, with the manufacturer's certification applied in accordance with 49 C.F.R. Sec. 571.218 indicating that the motorcycle helmet meets standards established by the United States department of transportation.
S5.5 Projections: A helmet shall not have any rigid projections inside its shell. Rigid projections outside any helmet's shell shall be limited to those required for operation of essential accessories, and shall not protrude more than 0.20 inch (5 mm).
That why I got a ticket for a Mohawk.