30 CFR 57.14132 - Horns and backup alarms for surface equipment.
(a) Manually-operated horns or other audible warning devices provided on self-propelled mobile equipment as a safety device shall be maintained in a functional condition.
(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.