[4] The NEPA was the first piece of legislation that created a comprehensive method to assess potential and existing environmental risks at once.
It also encourages communication and cooperation between all the actors involved in environmental decisions, including government officials, private businesses, and citizens.
[5] In particular, an EIS acts as an enforcement mechanism to ensure that the federal government adheres to the goals and policies outlined in the NEPA.
If the action may or may not cause a significant impact, the agency can first prepare a smaller, shorter document called an Environmental Assessment (EA).
A limited number of federal actions may avoid the EA and EIS requirements under NEPA if they meet the criteria for a categorical exclusion (CATEX).
NEPA does not prohibit the federal government or its licensees/permittees from harming the environment, instead it requires that the prospective impacts be understood and disclosed in advance.
The intent of NEPA is to help key decisionmakers and stakeholders balance the need to implement an action with its impacts on the surrounding human and natural environment, and provide opportunities for mitigating those impacts while keeping the cost and schedule for implementing the action under control.
Contrary to popular belief, the "No Action Alternative" doesn't necessarily mean that nothing will occur if that option is selected in the Record of Decision.
In that Record of Decision, the Texas Department of Transportation opted not to proceed with building its portion of I-69 as one of the Trans-Texas Corridors to be built as a new-terrain route (the Trans-Texas Corridor concept was ultimately scrapped entirely), but instead decided to proceed with converting existing US and state routes to I-69 by upgrading those roads to interstate standards.
Proceeding in this fashion helps avoid interagency conflicts and potential lawsuits after the lead agency reaches its decision.
In such cases, the Tier I EIS would analyze the potential socio-environmental impacts along a general corridor, but would not identify the exact location of where the action would occur.
For example, parts of the proposed Interstate 69 extension in Indiana and Texas, as well as portions of the Interstate 11 corridor in Nevada and Arizona are being studied through a two-tiered process By requiring agencies to complete an EIS, the act encourages them to consider the environmental costs of a project and introduces new information into the decision-making process.