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Tennessee Environmental Procedures Manual

Chapter 5: Impact Analysis

5.1 Types of Impacts

This section begins with a definition of the term, "impact." It then discusses the why (regulatory), who (applicable TDOT staff or consultant) and how (study and coordination process) of impact analyses conducted as part of the NEPA process for a proposed transportation project.

5.1.1 Definition of Impact

NEPA requires federal agencies to consider environmental issues prior to making any major decisions on federally-funded or permitted projects. To understand a project's potential benefit or harm to the environment, NEPA requires an assessment of potential impacts to the environment. Different types of impacts and different impact levels (i.e., significant or not significant) must be examined in this evaluation. As discussed in Chapter 3, the level of impact or potential impact is often the determining factor in the selection of the appropriate NEPA document for a project. Many of the technical areas have federal regulations and/or guidance that define "impact."  The term "effect" may be used interchangeably with "impact."

Three types of impacts are addressed under NEPA and many other environmental regulations:

  • Direct;
  • Indirect (or Secondary)1; and
  • Cumulative.

Table 5-1 shows the information that is fed into each type of impact analysis.

Table 5-1 Summary of Direct, Indirect and Cumulative Impacts
Type of Effect Direct Indirect Cumulative
Nature of effect Typical, inevitable, predictable Reasonably foreseeable, probable Reasonably foreseeable, probable
Cause of effect Project Project's direct and indirect effects Project's direct and indirect effects as well as the effects of other project-related activities
Timing of effect Project construction and implementation At some future time after direct effects Project construction or in the future
Location of effect Within project impact area Within boundaries of systems affected by project Within boundaries of systems affected by project

Source: A Guidebook for Evaluating the Indirect Land Use and Growth Impacts of Highway Improvements, Final Report, APR 327, Oregon Department of Transportation and FHWA, April 2001.

The level of analysis should be commensurate with the project's impact potential. Indirect and cumulative impacts could even occur before the transportation project is built (i.e., real estate speculators initiating land use actions in anticipation of project construction). Direct Effects

As defined in the CEQ regulations (40 CFR 1508.8(a)), direct effects are those "which are caused by the action and occur at the same time and place."

Examples of Direct Effects

  • Property takes or displacements
  • Traffic noise
  • Alterations to the visual character
  • Changes in traffic circulation patterns or access
  • Filling or draining wetlands Indirect and Cumulative Impacts

Besides direct and observable effects, in compliance with NEPA and CEQ regulations (40 CFR 1508), the indirect and the cumulative impacts of a project must be determined along with the direct impacts. The degree to which indirect and cumulative impacts need to be addressed in a NEPA document depends on the potential for the impacts to be significant and will vary by resource, project type, geographic location and other factors. This issue should be addressed, particularly when preparing an EIS or an EA, with other agencies and the NEPA participants during early coordination activities or scoping. The issue of indirect and cumulative impacts can be discussed on a resource-by-resource basis, and/or discussed in a separate section in the Impacts Chapter.

The indirect and cumulative impact evaluation addresses more than multiple federal actions. The evaluation includes impacts of past, present and reasonably foreseeable future actions by everyone.

Reasonably Foreseeable

Courts have defined reasonably foreseeable as an action that is sufficiently likely to occur, that a person of ordinary prudence would take into account in making a decision.

The indirect and cumulative analysis focuses on impacts to the human communities as a result of the proposed project and anticipated land use and development trends. The basis of the analysis is local and regional comprehensive development plans and zoning regulations, which are supplemented by census data, aerial photography and interviews with local government. Typically, the sphere of influence is the area within a one-mile radius of the study area boundaries, i.e., the area of analysis for indirect and cumulative impacts is larger than the study area for direct impacts. The project's design year is used for the reasonably foreseeable future time frame since design year traffic is based on the area's future land use assumptions. The examination of indirect and cumulative consequences should focus on the functional relationships of resources within larger systems.

Guidance on assessing indirect and cumulative impacts can be found on FHWA's website. Particularly useful are FHWA's April 1992 Position Paper: Secondary and Cumulative Impact Assessment in the Highway Development Process, which can be found at and their January 2003 Interim Guidance Questions and Answers Regarding the Consideration of Indirect and Cumulative Impacts in the NEPA Process at

Indirect and cumulative effects, not as easily recognizable as direct effects, are described below.

Indirect Effects

Indirect impacts are defined by 40 CFR 1508.7 as impacts that may be caused by a project, but would occur in the future or outside the project area and are reasonably foreseeable. Indirect impacts may include growth-inducing effects and other effects related to changes in the pattern of land use, population density or growth rate and related effects on air and water and other natural systems.

Examples of Reasonably Foreseeable Actions or Projects

  • A project identified in a local or regional comprehensive land use plan;
  • A subdivision plat that has been filed with the local government, county or other plat-approving agency;
  • Population/development trends that are identified in local or regional comprehensive land use plans;
  • Planned transportation improvements by city or county governments;
  • Changes in traffic circulation patterns or access; and
  • Local or regional infrastructure projects that could impact resources (schools, hospitals, etc.).

Actions that are not usually considered reasonably foreseeable include those that are possible but not likely to occur, and actions that have little or no influence on the transportation decision.

Often, if a project does not have a direct effect on a resource, it will not have an indirect effect on that resource. Occasionally, however, a project may not have a direct effect but it will have an indirect effect. In general, highway projects most commonly result in indirect impacts to land use, community and economic resources, farmland, water resources, water quality, wetlands and terrestrial ecology.

Access control, or lack of it, is a key factor in assessing the potential for indirect impacts. Projects with uncontrolled access alternatives are more likely to result in indirect impacts. For those alternatives with access controls, the indirect impact focus is generally in the area of the intersections or interchanges.

Meetings with local and regional planners and other appropriate agencies are helpful in determining potential indirect impacts. Environmental resources that can be sensitive to induced change (i.e., indirect impacts) include the social and economic structure of a community, floodplains and area-wide water quality. Analysis of indirect impacts must include identification of outside development pressures to determine the ability of an area to survive the removal of housing, businesses and community services. The analysis also examines whether a community can absorb relocated residents and businesses in terms of social and economic disruption (available housing, public services affected, areas zoned for business use, etc.).

Cumulative Effects

Cumulative impacts are the combined effects of all past, present and reasonably foreseeable projects (not just the current project and not just highway projects) on a given resource (e.g. wetlands), regardless of who has built the project (includes developers, localities, etc., not just state departments of transportation or federal agencies).

Legal Definition of Cumulative Effects

The impacts on the environment that results from the incremental impact of the action when added to other past, present, and reasonably foreseeable future actions regardless of what agency (Federal or non-Federal) or person undertakes such other actions. Cumulative impacts can result from individually minor but collectively significant actions taking place over a period of time.

40 CFR 1508.7

For cumulative impacts, development that is occurring or slated to occur independent of the project must be identified. Cumulative actions include existing residential, commercial, industrial, agricultural and infrastructure land uses. Cumulative impacts also include anticipated and planned new growth as defined in the indirect impacts section above, as well as the proposed highway and other highway improvements connecting to the proposed highway. Cumulative effects may be undetectable when viewed in the individual context of direct and even indirect impacts, but nonetheless can add to other disturbances and eventually, lead to a measurable environmental change.

If a project will not cause direct or indirect impacts on a resource, it will not contribute to a cumulative impact on the resource. Level of Impact

Different technical areas define levels of impacts in different terms. For example, in noise, impacts are classified as "minor, moderate, or substantial." For historic resources, impacts are classified as "no historic properties affected, no adverse effect, or adverse effect." Projects can also have beneficial, as well as adverse impacts.

The determination of impact level must consider both the context and intensity of the impact. These terms are defined in Section 3.1.1, Significance. Regarding context, it is important to identify how sensitive the impacted resource is. For example, is it of national, regional, state or local significance? Is it a watershed versus a stream channel? Are a few houses affected or is a whole neighborhood affected?

Regarding intensity, how bad (or good) is the impact? For example, is public health or public safety involved? Is there a high degree of public controversy? Will the project affect a unique or unusual area? Will federally listed species be adversely impacted? Or, will the project have beneficial impacts?

It is important for a planner to use the correct terms when summarizing a technical study or preparing an impact analysis. It is also important to avoid loosely using the terms "significant" or "significantly" to describe impacts in both technical studies and the NEPA document. If an impact is determined to be significant, the determination must be supported by factual information.


1. The term "secondary impact" does not appear, nor is it defined in either the CEQ regulations or related CEQ guidance. However, the term is used in the FHWA's Position Paper: Secondary and Cumulative Impact Assessment in the Highway Project Development Process (April, 1992) but is defined with the CEQ definition of indirect impact (40 CFR 1508.8). FHWA has used the terms interchangeably. For purposes of this guidance, secondary and indirect impacts are used interchangeably.

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