(a) An automobile may carry only one passenger in excess of its registered capacity, except in the case of infants.
(b) No person shall ride on the fenders, running board, any exterior fitting, or on the outside of motor vehicles, busses or trucks or hang on the sides thereof. The driver of any such vehicle shall be held guilty as an accessory if he permits such conduct. This subsection does not apply to a mechanic engaged in testing the vehicle or to a policeman.