Guidance for Preparers of Cumulative Impact Analysis

This guidance for preparers of cumulative impact analyses includes the introductory information below, a guidance paper, and three issue papers:

Purpose and Background


This guidance addresses cumulative impact analyses for surface transportation projects in California that are subject to the National Environmental Policy Act (NEPA) or California Environmental Quality Act (CEQA). The Council on Environmental Quality (CEQ) regulations implementing NEPA require federal agencies to consider the potential cumulative impacts associated with a proposed action. Similarly, CEQA and the CEQA Guidelines require State agencies to consider, among other impacts, the potential cumulative effects associated with a proposed project. Performing cumulative impact analyses helps to implement the overall goal of NEPA and CEQA, which is to insert environmental considerations into the planning process as early as possible and provide a sound basis for government decision-making.


The Federal Highway Administration (FHWA) - the California Department of Transportation (Caltrans), and the U.S. Environmental Protection Agency (EPA) recognize the importance of considering cumulative impacts during the preparation of environmental documents. An interagency Work Group representing FHWA California Division, Caltrans, and the EPA Region IX developed this guidance to assist Caltrans practitioners (environmental staff, project managers, and consultants) responsible for preparing environmental documents pursuant to NEPA and CEQA. While FHWA, EPA, and other agencies nationwide have prepared other guidance papers on this subject, this guidance was prepared to address cumulative impact analysis for transportation projects in California.


This guidance will help practitioners to identify cumulative impacts and prepare an analysis that is sound and well documented. Further, this guidance will help practitioners prepare cumulative impact analyses in support of other project-related analytical requirements, such as the Section 404(b)(1) Guidelines Alternatives Analysis required by Section 404 of the federal Clean Water Act. While the analyses required by Section 404 are sometimes performed after a NEPA or CEQA document has been completed or for different purposes (public documents, U.S. Army Corps of Engineers Section 404 permit applications, etc.), the Work Group intends that meaningful data developed during the cumulative impact analysis for a NEPA or CEQA document can be used to support Section 404 determinations.


An assessment of cumulative impacts is an integral part of a NEPA environmental document (40 CFR 1508.7). Although a cumulative impact analysis may be presented as a separate chapter or section of a NEPA (or CEQA) document, it is developed concurrently with direct and indirect impact analyses associated with the proposed project.


The Work Group intends this guidance to be practical and flexible. Cumulative impact analyses may vary according to type and scale of the proposed project and the resources affected. The guidance provides several tools and approaches that can be applied, depending on the potential effects of the proposed project, the type or condition of resources under consideration, and the professional judgment of the practitioner performing the analysis.

To provide flexibility, the guidance is presented as five linked online documents with links to other resources and agency guidance:

  • Introduction – Provides background about the guidance, its intended audience and purpose.
  • Approach and Guidance – The backbone of the guidance, this paper provides an eight-step approach for identifying and assessing cumulative impacts.
  • Issue Paper: Defining Resource Study Areas – An issue paper that presents practical advice for identifying the geographic boundaries for each resource included in the cumulative impact analysis.
  • Issue Paper: Data Gathering – An issue paper that presents methods for identifying available data for the cumulative impact analysis, as well as steps to take when data are not available.
  • Issue Paper: CEQA Guidelines for Cumulative and Indirect Impacts – Provides a brief primer for Caltrans planners who must analyze cumulative impacts to fulfill only CEQA requirements.

In addition to the information provided in the five papers, the guidance offers links to more detailed references, manuals, and policy guidance documents related to cumulative impacts. Highlighted reference links provide more detailed discussions on specific topics.

Additional Reference Materials

On September 18, 2002, President George W. Bush signed Executive Order (EO) 13274, Environmental Stewardship and Transportation Infrastructure Project Reviews. This EO established an Interagency Task Force to advance environmental stewardship and streamlining efforts, to coordinate expedited transportation decision-making, and to address priority projects. The Task Force established an interagency Work Group on Indirect and Cumulative Impacts to evaluate this topic and identify opportunities where greater interagency coordination and collaboration could lead to improvements in the decision-making process for transportation projects. The EO Task Force Work Group released its Draft Baseline Report on March 15, 2005. The appendices of the Draft Baseline Report include an annotated bibliography of current guidance, annotations on case law, and other helpful materials.


The Work Group prepared this guidance for its colleagues—environmental professionals with varying degrees of expertise. The modular structure of the guidance provides flexibility so that analysts can refer to specific topics. To build a strong foundation for cumulative impact analysis, this guidance provides the following:

  • Definitions of terms fundamental to cumulative impact analysis;
  • A suggested step-by step approach for performing the analysis; and
  • Examples of best practices and tools to use in the analysis.

This guidance helps practitioners outline a cumulative impact analysis and organize research. It can help to identify the appropriate resources to analyze, define the geographic and temporal parameters of the analysis, select the appropriate methods needed to assess impacts on individual resources, make supportable impact findings, and determine if and when mitigation measures should be proposed. The material presented in the guidance paper and associated issue papers is meant to be used in conjunction with—but not substituted for—agency policies, regulations, and legal requirements. Each agency contributing to the guidance recognizes that the approach to cumulative impact analysis may vary widely depending on the nature and context of the project proposed, the affected resources, the extent of available data, and other factors. It also recognizes that the guidance may be updated to reflect new issues or challenges as they arise. Notwithstanding the project-appropriate variations in method and procedure, FHWA, Caltrans, and EPA Region IX agree with the advice presented in this guidance concerning content, methods, analytical approach, and cumulative impact analysis formats.

The agencies that developed this guidance are interested in your views on it. If you have comments or suggestions regarding the guidance or issue papers, please contact:

Peter Bond
Environmental Management Office
P.O. Box 942874, Mail Station 27
Sacramento, CA 94274-0001
Phone: 916-.651-8164
Fax: 916.653.7757

Roadmap to the Guidance

The interagency Work Group interviewed Caltrans planners who had recently prepared cumulative impact analyses. Working together, they identified the following considerations or issues associated with cumulative impact analyses:

  1. What resources should I include in the analysis? Refer to Step 1 of the Approach and Guidance.
  2. How should I establish a cumulative impact study area? Refer to Step 2 of the Approach and Guidance, and the Defining Resource Study Areas issue paper.
  3. Is information available to conduct the analysis and make valid conclusions? Refer to the Data Gathering issue paper, and Steps 3, 4 and 5 of the Approach and Guidance.
  4. How can I describe the current health and historical context for each resource? Refer to Step 3 of the Approach and Guidance.
  5. What is the best way to identify the “reasonably foreseeable” projects to include in the analysis? Refer to Step 5 of the Approach and Guidance and the Data Gathering issue paper.
  6. What method should I use to perform the analysis? When should I apply quantitative or qualitative analysis techniques? Refer to Step 6 of the Approach and Guidance.
  7. How should I report my conclusions? Refer to Step 7 of the Approach and Guidance.
  8. How should I address mitigation? Refer to Step 8 of the Approach and Guidance.