30 C.F.R. § 56.14103 – Operators stations.

(a) If windows are provided on operators' stations of self-propelled mobile equipment, the windows shall be made of safety glass or material with equivalent safety characteristics. The windows shall be maintained to provide visibility for safe operation.
(b) If damaged windows obscure visibility necessary for safe operation, or create a hazard to the equipment operator, the windows shall be replaced or removed. Damaged windows shall be replaced if absence of a window would expose the equipment operator to hazardous environmental conditions which would affect the ability of the equipment operator to safely operate the equipment.
(c) The operator's stations of self-propelled mobile equipment shall—
1. (1) Be free of materials that could create a hazard to persons by impairing the safe operation of the equipment; and
2. (2) Not be modified, in a manner that obscures visibility necessary for safe operation.