Which agency is primarily responsible for regulating HAZMAT transportation in the United States?

The Department of Transportation (DOT) is the primary agency responsible for regulating hazardous materials (HAZMAT) transportation in the United States. It governs and oversees the safe transport of hazardous materials through various modes, including road, rail, air, and water, by establishing a set of regulations and standards. These regulations are detailed under the Hazardous Materials Regulation (HMR) as outlined in Title 49 of the Code of Federal Regulations (CFR).

DOT's role is critical in ensuring public safety, environmental protection, and the wellbeing of transport workers when dealing with hazardous materials. The agency develops guidelines that define how different types of hazardous materials should be packaged, labeled, and transported to minimize risks during transit.

Other agencies such as the Environmental Protection Agency and the Occupational Safety and Health Administration have important roles in environmental protection and worker safety, respectively, but their mandates do not focus specifically on transport regulation like DOT does. The National Transportation Safety Board is primarily focused on investigating transportation accidents rather than regulating HAZMAT transportation. Thus, DOT's comprehensive oversight makes it the correct answer regarding who regulates hazardous materials transportation.