MSHA - Code of Federal Regulations - 30 CFR 77.1604
30 CFR § 77.1604
Transportation of persons; overcrowding.
(a) No man-trip vehicle or other conveyance used to transport persons
to and from work areas at surface coal mines shall be overcrowded and all
persons shall ride in a safe position.
(b) Supplies, materials, and tools other than small handtools shall
not be transported with men in man-trip vehicles unless such vehicles are
specifically designed to make such transportation safe.