The Department of Transportation (DOT) was established by an act of Congress on October 15, 1966. The Department’s first official day of operation was April 1, 1967. [http://www.dot.gov/about.html]

The mission of the Department of Transportation is to ensure a fast, safe, efficient, accessible and convenient transportation system that meets our vital national interests and enhances the quality of life of the American people.

Organizations within the DOT include the Federal Highway Administration, the Federal Aviation Administration, the National Highway Traffic Safety Administration, the Federal Transit Administration, the Federal Railroad Administration and the Maritime Administration.

For further information about the functions, organization, and activities, of the U.S. Department of Transportation, please visit http://www.dot.gov/.

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Source:
http://www.whitehouse.gov/our-government/executive-branch