Md. Code, Transportation § 21–1131

(a) Except as provided in subsection (b) of this section, a person may not knowingly or intentionally cause a diesel–powered motor vehicle to discharge clearly visible smoke, soot, or other exhaust emissions onto another person or motor vehicle.

(b) This section does not apply to a person operating:

(1) A diesel–powered vehicle that discharges visible exhaust as the result of normal acceleration or towing;

(2) A commercial vehicle with a gross weight of 10,000 pounds or more; or

(3) A construction vehicle operating at a construction site.