Which regulatory agency sets standards for the transport of hazardous materials in the United States?

The Department of Transportation (DOT) is the agency responsible for establishing and enforcing regulations regarding the transport of hazardous materials in the United States. This includes overseeing the safe and efficient movement of hazardous substances by road, rail, air, and waterways. The DOT's Pipeline and Hazardous Materials Safety Administration (PHMSA) specifically manages the hazardous materials regulations (HMR), which cover packaging, labeling, and the duties of transportation companies involved in the hazardous materials shipping process.

The other agencies do play important roles in environmental protection, workplace safety, or specific industries but do not directly address the transportation of hazardous materials. For example, the Environmental Protection Agency (EPA) focuses on environmental health and regulates hazardous waste, while the Nuclear Regulatory Commission (NRC) oversees nuclear materials and safety. The Occupational Safety and Health Administration (OSHA) is primarily concerned with workplace safety and health regulations. Therefore, the DOT's dedicated focus on the transportation of hazardous materials establishes it as the correct answer in this context.