The Hazardous Material Regulations at 49 CFR 172.516 require the following for the display of placards.

• On a motor vehicle or rail car a placard must be clearly visible from the direction it faces, except from the direction of another rail cargo or transport vehicle to which it is attached.  More…
• In all cases placards must be…
  • Securely attached or placed in a holder.
  • Located clear of ladders, pipes, doors, and tarps.
  • So far as practicable, located so that dirt or water is not directed to it from the wheels.
  • Located away (≥3 inches) from any marking (such as advertising) that could substantially reduce its effectiveness, i.e. make it harder to see.
  • Displayed horizontally so words or identification numbers read left to right.
  • Maintained by the carrier so no substantial reduction in visibility.
  • Affixed to a background of contrasting color or have dotted or solid line outer border which contrasts with the background color.
• Recommended specifications for a placard holder found in Appendix C to 49 CFR part 172.
• Placard or placard holder may be hinged if format, color, and legibility of the placard are maintained.

BUT WHAT ABOUT AT NIGHT?

Compliance with 49 CFR 172.516 won’t help the visiblity of your placards at all after dark.  But someone has come up with a good idea that I wish to share with you here.  Go Glow Placards, LLC makes an LED lit placard holder that will ensure 24 hour visibility of the placards you display.  I think it will also heighten overall safety as other drivers are more likely to notice placards displayed in the Go Glow Placard Holder.

Compliance is important but it may be just the starting point.  Be safe and be seen.