MSHA - Code of Federal Regulations - 30 CFR 56.14132
30 CFR § 56.14132
Horns and backup alarms.
(a) Manually-operated horns or other audible warning devices provided
on self-propelled mobile equipment as a safety feature shall be maintained in
(b)(1) When the operator has an obstructed view to the rear,
self-propelled mobile equipment shall have
(b)(1)(i) An automatic reverse-activated signal alarm;
(b)(1)(ii) A wheel-mounted bell alarm which sounds at least
once for each three feet of reverse movement;
(b)(1)(iii) A discriminating backup alarm that covers the
area of obstructed view; or
(b)(1)(iv) An observer to signal when it is safe to back
(b)(2) Alarms shall be audible above the surrounding noise
(b)(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.