Tanker Safety Improvement Act - Requires, under the Ports and Waterways Safety Act of 1972, that vessels subject to such Act shall be constructed and operated using the best available pollution prevention technology.

Requires vessels of more than 20,000 deadweight tons which are documented under the laws of the United States and engaged in the carriage of oil in bulk to ports of the United States situated on internal waters and straits, if the vessel's construction is contracted for or begun after June 30, 1975, to be equipped with a segregated ballast capacity.