Final answer:
The frequency of required special training for drivers transporting hazardous materials like radioactive substances or flammable cryogenic liquids is determined by specific regulations from the United States Department of Transportation and the EPA, and may vary.
Step-by-step explanation:
The United States Department of Transportation and the Environmental Protection Agency have specific regulations concerning the transportation of hazardous materials, which include guidelines on the necessary training for drivers transporting radioactive material or flammable cryogenic liquids.
These regulations mandate that drivers must undergo specialized training to ensure the safe handling and transportation of such potentially dangerous substances. However, the actual requirement for the frequency of this training may vary depending on the specific regulations and guidelines in place, which are subject to change and are often detailed in industry and government publications or through official training programs.