30 C.F.R. § 56.14132 Horns and backup alarms.
Title 30 - Mineral Resources
(a) Manually-operated horns or other audible warning devices provided on self-propelled mobile equipment as a safety feature shall be maintained in functional condition. (b)(1) When the operator has an obstructed view to the rear, self-propelled mobile equipment shall have— (i) An automatic reverse-activated signal alarm; (ii) A wheel-mounted bell alarm which sounds at least once for each three feet of reverse movement; (iii) A discriminating backup alarm that covers the area of obstructed view; or (iv) An observer to signal when it is safe to back up. (2) Alarms shall be audible above the surrounding noise level. (3) An automatic reverse-activated strobe light may be used at night in lieu of an audible reverse alarm. (c) This standard does not apply to rail equipment.
Title 30: Mineral Resources
PART 56—SAFETY AND HEALTH STANDARDS—SURFACE METAL AND NONMETAL MINES
Subpart M—Machinery and Equipment
Safety Devices and Maintenance Requirements
§ 56.14132 Horns and backup alarms.