Which federal agency primarily writes the hazardous materials regulations for transportation?

The US Department of Transportation, through the Pipeline and Hazardous Materials Safety Administration (PHMSA), writes and enforces the Hazardous Materials Regulations found in 49 CFR Parts 100-185. EPA regulates hazardous waste and environmental impacts (separate but overlapping rules), OSHA regulates workplace safety including chemical exposure for employees, and DHS regulates hazmat as a security matter (including the security threat assessment required for the CDL hazmat endorsement). PHMSA's rules govern packaging, labeling, placarding, shipping papers, training, and incident reporting — basically everything about how a hazmat shipment travels from origin to destination.