Regulations that implement the National Environmental Policy Act (NEPA) encourage Environmental Impact Statements (EISs) to be performed in tiers or stages (40 C.F.R. 1502.20). Tier I of an EIS would typically examine a range of alternatives at a transportation corridor or system level. The current emphasis on environmental stewardship and environmental streamlining is resulting in more frequent use of Tier I EISs early in the transportation project planning process. NEPA and its implementing regulations, however, provide little guidance about when the preparation of Tier 1 and Tier 2 EISs is appropriate, the level of analyses needed, and the tools necessary to perform the analyses. The objective of this study was to develop guidelines for state Departments of Transportation on the use of tiered environmental impact statements.