Md. Code, Transportation § 21–1107

(a) Subject to subsection (c) of this section, this section applies to:

(1) A truck that has a gross vehicle weight rating of 10,001 pounds or more; and

(2) A truck/trailer or truck tractor/semitrailer or trailer combination that has a combined gross vehicle weight rating of 10,001 pounds or more.

(b) While a vehicle subject to this section is being operated on a highway:

(1) The driver may not allow a person to occupy the area of the vehicle primarily intended to carry cargo; and

(2) A person may not occupy the area of the vehicle primarily intended to carry cargo.

(c) This section does not apply to:

(1) An attendant delegated to care for livestock;

(2) A vehicle that is controlled or operated by a farmer and is:

(i) Used to transport agricultural products, farm machinery, or farm supplies to or from a farm;

(ii) Not used in the operations of a common or contract motor carrier; and

(iii) Used within 150 miles of the farmer’s farm;

(3) A vehicle owned or operated by the U.S. Department of Defense if the vehicle is controlled or operated by:

(i) Active duty military personnel; or

(ii) 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

(4) A vehicle traveling at a speed of not more than 25 miles per hour.