30 C.F.R. § 56.14207 : Mineral Resources — MINE SAFETY AND HEALTH ADMINISTRATION, DEPARTMENT OF LABOR — METAL AND NONMETAL MINE SAFETY AND HEALTH — SAFETY AND HEALTH STANDARDS-SURFACE METAL AND NONMETAL MINES — MACHINERY AND EQUIPMENT —SAFETY PRACTICES AND OPERATIONAL PROCEDURES — Parking procedures for unattended equipment

Mobile equipment shall not be left unattended unless the controls are placed in the park position and the parking brake, if provided, is set. When parked on a grade, the wheels or tracks of mobile equipment shall be either chocked or turned into a bank.

30 C.F.R. §56.14207