skip navigation
print icon Print

Indirect Effects/Cumulative Impacts


This section provides a brief overview of secondary/indirect and cumulative impacts and related requirements as they affect the transportation community. Topics include the following:


The Federal Highway Administration and other federal agencies’ responsibility to address and consider direct, indirect, and cumulative impacts in the National Environmental Policy Act (NEPA) process was established in the Council on Environmental Quality (CEQ) Regulations for Implementing the Procedural Provisions of the National Environmental Policy Act (40 CFR §§1500-1508).

The CEQ regulations define the impacts and effects that must be addressed and considered by federal agencies in satisfying the requirements of the NEPA process. Direct, indirect and cumulative impacts can be defined as follows:

  • Direct effects are caused by the action and occur at the same time and place. (40 CFR § 1508.8)
  • Indirect effects are caused by the action and are later in time or farther removed in distance, but are still reasonably foreseeable. Indirect effects may include growth inducing effects and other effects related to induced changes in the pattern of land use, population density or growth rate, and related effects on air and water and other natural systems, including ecosystems. (40 CFR § 1508.8) Indirect effects and secondary effects are used interchangeably by FHWA.
  • Cumulative impact is the impact on the environment, which 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)

According to FHWA guidance, the determination or estimation of future impacts is essential to both indirect and cumulative impact analysis. However, the focus must be on reasonably foreseeable actions, those that are likely to occur or probable, rather than those that are merely possible.

For additional information on indirect/secondary and cumulative impacts for transportation projects, please see the question-and-answer guidance document, FHWA Interim Guidance: Indirect and Cumulative Impacts in NEPA [DOC 186kb].

[back to top]

Indirect Effects

Indirect effects (effects caused by the project, but occurring later in time or farther removed in distance than direct impacts) include changes in land use attributable to the project (induced growth) and impacts on environmental resources that occur as a result of the project’s influence on land use. Besides induced growth and related effects, the definition of indirect effects also includes other potential environmental impacts caused by a project, such as the effect of habitat fragmentation on species viability over time or changes in wetland functions due to stormwater runoff.

Induced Growth and Land Use Change

Transportation projects often reduce travel time, enhancing the attractiveness of surrounding land for development through changes in accessibility. These changes in accessibility may influence development in a localized area adjacent to the transportation project (e.g. gas stations and motels near an interchange), as well as larger-scale effects on the location of future development within a region. However, transportation projects are only one of numerous factors that influence development patterns. Other important factors include land availability and prices, economic conditions, political and regulatory conditions, and the quality of public services. The estimation of induced growth effects requires the identification of the project contribution to changes in development patterns. Once the project effect on land use has been identified, this information can be used to estimate the environmental impacts attributable to land use changes caused by the project, such as habitat fragmentation or stormwater runoff effects on water quality.

Methods for analyzing induced growth effects of transportation projects include quantitative methods, such as travel demand models and integrated land use and transportation models, and qualitative methods such as scenario writing, Delphi Technique, and expert panels.

[back to top]

Cumulative Impacts

According to the FHWA’s Interim Guidance: Questions and Answers Regarding the Consideration of Indirect and Cumulative Impacts in the NEPA Process (2003), cumulative impacts include the total of all impacts to a particular resource that have occurred, are occurring, and will likely occur as a result of any action or influence, including the direct and reasonably foreseeable indirect impacts of a Federal activity. According to the guidance:

“Cumulative impact analysis is resource specific and generally performed for the environmental resources directly impacted by a Federal action under study, such as a transportation project. However, not all of the resources directly impacted by a project will require a cumulative impact analysis. The resources subject to a cumulative impact assessment should be determined on a case-by-case basis early in the NEPA process, generally as part of early coordination or scoping.”

CEQ guidance titled Considering Cumulative Effects Under the National Environmental Policy Act identifies four basic types of effects that can lead to cumulative impacts:

  • Type 1: Repeated additive effects on a resource from a single project.
  • Type 2: Stressors from a single project that have interactive (countervailing or synergistic) net effect on a resource.
  • Type 3: Additive effects on a resource arising from multiple sources (projects, point sources, or general effects associated with development).
  • Type 4: Effects arising from multiple sources that affect resources in an interactive fashion.

Cumulative impact analysis requires careful development of the no build alternative scenario to include reasonably foreseeable future actions. Coordination with agency and local officials is an important part of identifying other projects that should be included as part of the no build alternative.

CEQ guidance identifies methods for analyzing cumulative effects, including: questionnaires, interviews and panels, checklists, matrices, networks and system diagrams, modeling, trends analysis, overlay mapping and geographic information systems, carrying capacity analysis, ecosystem analysis, economic impact analysis and social impact analysis.

[back to top]

The Indirect Effects and Cumulative Impacts Analysis Process for Transportation Projects

Separate analyses should be conducted to evaluate indirect effects of transportation projects and to evaluate project-related cumulative impacts. However both processes should include consultation with stakeholders and the public, identification of important trends and issues, and analysis of the potential for land use change and related environmental impacts on valued and vulnerable resources.

As summarized below, the National Cooperative Highway Research Program (NCHRP) has provided a detailed step-by-step approach for conducting indirect effect assessment, and the CEQ guidance outlined steps for cumulative impact assessments.

Indirect Effects Analysis

Specifically, for evaluating indirect effects, NCHRP Report 466: Desk Reference for Estimating the Indirect Effects of Proposed Transportation Projects presents the following eight-step process:

  • Step 1 – Define the Study Area Boundaries. The purpose of Step 1 is to set appropriate study area boundaries for the analysis of indirect effects as well as the timeframe for the analysis.
  • Step 2 – Identify the Study Area Communities’ Trends and Goals. The purpose of Step 2 is to gather information on community trends and goals in the study area, focusing on socioeconomic and land use issues.
  • Step 3 – Inventory Notable Features. The purpose of Step 3 is to identify specific valued, vulnerable or unique elements of the natural environment that will be analyzed in the assessment of indirect effects.
  • Step 4 – Identify Impact Causing Activities of the Proposed Action and Alternatives. The purpose of Step 4 is to identify the cause and effect relationships between the transportation project and potential impacts that may come into conflict with the goals identified in Step 2 or the notable features identified in Step 3.
  • Step 5 – Identify Potential Impacts For Analysis. The purpose of Step 5 is to compare the impact-causing activities developed in Step 4 with the inventory of goals, trends and notable features that make up the baseline conditions identified in Steps 2 and 3.
  • Step 6 – Analyze Impacts. The purpose of Step 6 is to determine the magnitude and location of the potential impacts identified in Step 5.
  • Step 7 – Evaluate Analysis Results. The purpose of Step 7 is to evaluate the uncertainties in the methodology used to evaluate impacts, in order to better understand the analysis results.
  • Step 8 – Assess Consequences and Develop Mitigation. The purpose of Step 8 is to assess the consequences of the impacts and to develop strategies to address unacceptable impacts, which occur when an impact identified in Step 6 conflicts with a goal identified in Step 2 or with a notable feature identified in Step 3.

Cumulative Impact Assessment

Although not specific to transportation projects, CEQ provides the following 11-step process for cumulative impact assessment in the report Considering Cumulative Effects Under the National Environmental Policy Act:

  • Step 1 – Identify the significant cumulative effects issues associated with the proposed action and define the assessment goals.
  • Step 2 – Establish the geographic scope for the analysis.
  • Step 3 – Establish the time frame for the analysis.
  • Step 4 – Identify other actions affecting the resources, ecosystems, and human communities of concern.
  • Step 5 – Characterize the resources, ecosystems, and human communities identified during scoping in terms of their response to change and capacity to withstand stresses.
  • Step 6 – Characterize the stresses affecting these resources, ecosystems, and human communities and their relation to regulatory thresholds.
  • Step 7 – Define a baseline condition for the resources, ecosystems, and human communities.
  • Step 8 – Identify the important cause and-effect relationships between human activities and resources, ecosystems, and human communities.
  • Step 9 – Determine the magnitude and significance of cumulative effects.
  • Step 10 – Modify or add alternatives to avoid, minimize, or mitigate significant cumulative effects.
  • Step 11 – Monitor the cumulative effects of the selected alternative and adapt management.

The NCHRP and CEQ approaches involve overlapping steps that can be conducted concurrently and support both indirect effect and cumulative impact processes, such as establishing the boundary and baseline conditions and trends for the resources of concern. However, care should be taken when documenting the IECI assessment results in the environmental assessment or environmental impact statement to clearly differentiate between indirect effects and cumulative impacts and, thereby, ensure that both topics are sufficiently addressed. One way to do this is to discuss indirect effects and cumulative impacts in separate sections within a single chapter, or as separate chapters (See Case Studies- St. Croix River Crossing Supplemental Final Environmental Impact Statement (SFEIS)), to make sure that each can stand on its own as a hard look at the pertinent issues.

Generally, the resources of concern for cumulative impact evaluation are those for which the project has the potential to directly or indirectly effect. Both the NCHRP and CEQ methodologies allow for either quantitative, e.g., analytical model, or qualitative, e.g., expert panel, analysis of the magnitude of the effects on the resources of concern. For complex projects, it is often advisable to employ more than one analysis tool to improve the confidence in the analytical results and/or to reveal a range of results.

[back to top]

Featured Case Study

  • Case Study Photo

    Texas DOT's indirect and cumulative impacts guidance documents help streamline implementation of projects such as FM 2493 EA Project in Smith County.

    Read Case Study >Photo: Texas DOT

View All Case Studies for this Topic >

New - items posted in the last 7 days

AASHTO (American Association of State Highway and Transportation Officials)
skip navigation