§ 509-k. Ill or fatigued operator. No driver shall operate a bus and a motor carrier shall not permit a driver to operate a bus while the driver's ability or alertness is so impaired, or so likely to become impaired, through fatigue, illness or any other cause, as to make it unsafe for him to begin or continue to operate the bus. At the request of the driver or the motor carrier such illness, fatigue, or other cause shall be certified by a qualified physician. However, in a case of grave emergency where the hazard to occupants of the bus or other users of the highway would be increased by compliance with this section, the driver may continue to operate the bus to the nearest place at which that hazard is removed.