There are three classes of actions which prescribe the level of documentation required in the NEPA process.
(a) Class I (EISs). Actions that significantly affect the environment require an EIS (40 CFR 1508.27). The following are examples of actions that normally required an EIS:
(1) A new controlled access freeway.
(2) A highway project of four or more lanes on a new location.
(3) Construction or extension of a fixed transit facility (e.g., rapid rail, light rail, commuter rail, bus rapid transit) that will not be located within an existing transportation right-of-way.
(4) New construction or extension of a separate roadway for buses or high occupancy vehicles not located within an existing highway facility.
(b) Class II (CEs). Actions that do not individually or cumulatively have a significant environmental effect are excluded from the requirement to prepare an EA or EIS. A specific list of CEs normally not requiring NEPA documentation is set forth in § 771.117(c) for FHWA actions or pursuant to § 771.118(c) for FTA actions. When appropriately documented, additional projects may also qualify as CEs pursuant to § 771.117(d) for FHWA actions or pursuant to § 771.118(d) for FTA actions.
(c) Class III (EAs). Actions in which the significance of the environmental impact is not clearly estabilished. All actions that are not Class I or II are Class III. All actions in this class require the preparation of an EA to determine the appropriate environmental document required.