339.14  DOORS ON BUSES.
   (a)   Except as provided by Section 331.40, all buses shall be provided with suitable gates or doors which shall be kept closed while such bus is in motion, and opened only when necessary to load and discharge passengers, and such doors shall not be reopened at the same stop after such bus has been put in motion.
   (b)   Whoever violates any provision of this section is guilty of a minor misdemeanor on a first offense; on a second offense within one year after the first offense, the person is guilty of a misdemeanor of the fourth degree; on each subsequent offense within one year after the first offense, the person is guilty of a misdemeanor of the third degree.