Except as provided in s. 346.595 (3m), no more than 2 persons may ride on a motorcycle during operation, and then only if the vehicle is equipped and designed with adequate seats and foot rests or pegs. Foot rests or pegs shall be mounted in accordance with manufacturer’s specifications. In the absence of manufacturer’s specifications, foot rests or pegs for the passenger shall be located on the same horizontal plane as those of the operator.