DOT employs almost 60,000 people across the country, in the Office of the Secretary of Transportation (OST) and its operating administrations and bureaus, each with its own management and organizational structure:
Office of the Secretary of Transportation (OST)
National Highway Traffic Safety Administration (NHTSA)
Federal Aviation Administration (FAA)
Office of Inspector General (OIG)
Federal Highway Administration (FHWA)
Pipeline and Hazardous Materials Safety Administration (PHMSA)
Federal Motor Carrier Safety Administration (FMCSA)
Research and Innovative Technology Administration (RITA)
Federal Railroad Administration (FRA)
Saint Lawrence Seaway Development Corporation (SLSDC)
Federal Transit Administration (FTA)
Surface Transportation Board (STB)
Maritime Administration (MARAD)

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United States Department of Transportation -- Publications & Papers