What Does 'No HM' Mean On A Road Sign?
Just when you think you've seen all the weirdest road signs out there, you come across one with the letters HM inside a red circle crossed by a slash. The mind reels at the sheer number of different word combinations that HM could mean... and that's before you even start to consider why whatever HM stands for would be prohibited on the route you're taking. Per the Manual on Uniform Traffic Control Devices (MUTCD), HM is shorthand for hazardous materials. Signs with HM crossed out are letting drivers know that the road can't be used to transport hazardous materials.
These No HM signs are reserved specifically for roads where a proper authority has determined that, for whatever reason, transport of certain hazardous materials should be kept off the route. Truckers carrying flammable, poisonous, corrosive, or otherwise dangerous substances would then need to find an alternate route. Federal roadway sign standards say that No HM signs should be placed where the driver would have an opportunity to choose an alternate route.
Other versions of the No HM sign
You might also see signs with the letters HM inside a green circle. Roads with these signs are designated as approved routes for hazardous-material transport, according to the MUTCD. Restrictions can be used in densely populated areas where an accident involving a vehicle carrying hazardous materials could put people, businesses, and the surrounding environment at risk.
The HM designation replaced an older HC marking, which stood for Hazardous Cargo. The change was made at the federal level in the MUTCD in 2003. However, regulations can vary from one state or jurisdiction to another. This can definitely make for some confusing traffic signs. For instance, some other versions of the sign you might encounter are a white Hazmat Carriers Prohibited sign, a red Hazardous Materials Prohibited sign, or a white bar with No Hazmats on an otherwise green highway distance sign. Some also pair the sign with a black propane tank-shaped symbol with a red strikethrough over it.