The National Environmental Policy Act (42 U.S.C. §4321 et seq) requires federal agencies to assess the environmental impact of all major federal actions significantly affecting the quality of the human environment. An environmental assessment provides evidence for determining whether a proposed agency action would require preparation of an Environmental Impact Statement (EIS) or a finding of no significant impact. If an EIS is required:
Environmental Impact Statements often are available on the websites of the U.S. government agencies that prepared them. But more comprehensive collections are readily available, specifically the EPA Environmental Impact Statement Database and the Northwestern University Transportation Library.