49 CFR § 393.24 - Requirements for head lamps, auxiliary driving lamps and front fog lamps.
(a)Headlamps. Every bus, truck and truck tractor shall be equipped with headlamps as required by § 393.11(a). The headlamps shall provide an upper and lower beam distribution of light, selectable at the driver's will and be steady-burning. The head lamps shall be marked in accordance with FMVSS No. 108. Auxiliary driving lamps and/or front fog lamps may not be used to satisfy the requirements of this paragraph.
(b)Auxiliary driving lamps and front fog lamps.Commercial motor vehicles may be equipped with auxiliary driving lamps and/or front fog lamps for use in conjunction with, but not in lieu of the required head lamps. Auxiliary driving lamps shall meet SAE Standard J581 Auxiliary Upper Beam Lamps, July 2004, and front fog lamps shall meet SAE Standard J583 Front Fog Lamp, August 2004. (See § 393.7 for information on the incorporation by reference and availability of these documents.)
(c)Mounting. Headlamps shall be mounted and aimable in accordance with FMVSS No. 108. Auxiliary driving lamps and front fog lamps shall be mounted so that the beams are aimable and the mounting shall prevent the aim of the lighting device from being disturbed while the vehicle is operating on public roads.