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. (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
up. (b)(2) Alarms shall be audible above the surrounding noise
level. (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.
