49 C.F.R. § 193.2917 – Warning signs.

(a) Warning signs must be conspicuously placed along each protective enclosure at intervals so that at least one sign is recognizable at night from a distance of 30m (100 ft.) from any way that could reasonably be used to approach the enclosure.
(b) Signs must be marked with at least the following on a background of sharply contrasting color:
The words “NO TRESPASSING,” or words of comparable meaning.

[Amdt. 193-2, 45 FR 70409, Oct. 23, 1980, as amended at 47 FR 32720, July 29, 1982]