MSHA - Code of Federal Regulations - 30 CFR 57.9305
30 CFR § 57.9305
(a) If truck spotters are used, they shall be in the clear while
trucks are backing into dumping position or dumping.
(b) Spotters shall use signal lights to direct trucks where
visibility is limited.
(c) When a truck operator cannot clearly recognize the spotter's
signals, the truck shall be stopped.