Subject to subsection (c) of this section, this section applies to: A truck that has a gross vehicle weight rating of 10,001 pounds or more; and A truck/trailer or truck tractor/semitrailer or trailer combination that has a combined gross vehicle weight rating of 10,001 pounds or more. While a vehicle subject to this section is being operated on a highway: The driver may not allow a person to occupy the area of the vehicle primarily intended to carry cargo; and A person may not occupy the area of the vehicle primarily intended to carry cargo. This section does not apply to: An attendant delegated to care for livestock; A vehicle that is controlled or operated by a farmer and is: Used to transport agricultural products, farm machinery, or farm supplies to or from a farm; Not used in the operations of a common or contract motor carrier; and Used within 150 miles of the farmer’s farm; A vehicle owned or operated by the U.S. Department of Defense if the vehicle is controlled or operated by: Active duty military personnel; or A member of the military reserves or National Guard on active duty, including personnel on full-time National Guard duty and personnel on part-time training; or A vehicle traveling at a speed of not more than 25 miles per hour.