49 CFR 385.403: Prohibited transportation without safety permit.

A motor carrier may not transport in interstate or intrastate commerce any of the following hazardous materials, in the quantity indicated for each, unless the motor carrier holds a safety permit:

(a) A highway route-controlled quantity of a Class 7 (radioactive) material, as defined in § 173.403 of this title;

(b) More than 25 kg (55 pounds) net weight of a Division 1.1, 1.2, or 1.3 (explosive) material or articles or an amount of a Division 1.5 (explosive) material requiring placarding under part 172 of this title;

(c) More than one liter (1.08 quarts) per package of a “material poisonous by inhalation,” as defined in § 171.8 of this title, that meets the criteria for “hazard zone A,” as specified in § 173.116(a) or § 173.133(a) of this title;

(d) A “material poisonous by inhalation,” in a “bulk packaging,” both as defined in § 171.8 of this title, that meets the criteria for “hazard zone B,” as specified in § 173.116(a) or § 173.133(a);

(e) A “material poisonous by inhalation,” as defined in § 171.8 of this title, that meets the criteria for “hazard zone C,” or “hazard zone D,” as specified in § 173.116(a) of this title, in a packaging having a capacity equal to or greater than 13,248 L (3,500) gallons; or

(f) A shipment of methane (compressed or refrigerated liquid), natural gas (compressed or refrigerated liquid), or any other compressed or refrigerated liquefied gas with a methane content of at least 85 percent, in bulk packaging having a capacity equal to or greater than 13,248 L (3,500 gallons).