Indirect effects and cumulative impacts

Learn how to prepare indirect effects and cumulative impacts analyses for projects requiring an Environmental Assessment (EA) or an Environmental Impact Statement (EIS).

The Washington State Department of Transportation (WSDOT) recommends indirect effects are addressed alongside direct impacts in the environmental (EA or EIS) document. We recommend preparing a separate cumulative impact analysis.

Before you begin your analyses, reference the following guidance documents:

  • Chapter 412: Indirect and Cumulative Impacts of our Environmental Manual provides policy direction and important Washington State Department of Transportation (WSDOT) guidance on indirect effects, cumulative impacts, and climate change as a cumulative effect.
  • Guidance on Preparing Cumulative Impact Analyses (pdf 150 kb) – This document presents the methods to conduct these analyses. This joint guidance between WSDOT, the Federal Highway Administration (FHWA), and the Environmental Protection Agency (EPA) provides the eight-step process that should be followed for WSDOT projects.

Indirect effects analysis process

Indirect effects are caused by the project or plan, but are separated from direct effects by time or distance. Indirect effects include induced growth and related environmental impacts. To avoid confusion when designing your analysis, we advise the consistent use of the phrase: “indirect effects of the project on [resource].”

The NEPA project lead provides the authors of individual discipline studies with a project summary that identifies whether the project purpose is to encourage economic development or changes in land use.

Analysts evaluate the likelihood of new developments or environmental conditions in the project area following project construction. Look for the “but-for” relationship: that is, the development would not occur but for the transportation project. Another example is a future loss of wetlands resulting from changes in development caused by the project.

Cumulative impacts analysis process

Cumulative impacts are the aggregate result of incremental direct and indirect effects of a project or plan, effects of past and present actions, and effects of other reasonably foreseeable future actions on resources of concern. To avoid confusion when designing your analysis, we advise the consistent use of the phrase: “the project’s contribution to cumulative impacts on [resource].”

The NEPA project lead determines the appropriate method for documenting cumulative impacts. Options are to:

  1. Prepare a separate chapter or section on cumulative effects.
  2. Integrate the disclosure of cumulative impacts within the individual discipline studies and report within those chapters or sections. In this case, a separate cumulative impacts Discipline Report or tech memo is not required.

Analysts compile information on trends for each resource before establishing a list of reasonably foreseeable future projects. Assess whether any direct or indirect effects from the project, combined with what we know about the future, will change the trend. This change may be either positive or negative, short-term or long-term. If writing directly to the EA/EIS, carefully and fully cite information sources.

Visit WSDOT’s Climate Change webpage for more guidance on considering climate change as a cumulative impact.

Example documents

Refer to the SR 502 Corridor Widening Final Indirect and Cumulative Effects Analysis (pdf 1.34 mb) and the EA for Joint Base Lewis McCord Indirect and Cumulative Effects Analysis (pdf 414 kb) as helpful examples when planning your own analysis. 

WSDOT staff can get more examples of indirect and cumulative analyses through our internal Sample Document Library. Contact Victoria Book at for access.

External references

AASHTO’s Indirect Effects/Cumulative Impacts website provides case studies, helpful tips, and lessons learned from transportation projects nationwide.

AASHTO Practitioner's Handbook 12: Assessing Indirect Effects and Cumulative Impacts Under NEPA (2016) (pdf 1.5 mb) provides practical tips for NEPA project leads and analysts. It contains tips, key questions to answer, steps (similar to WSDOT’s), and legal adequacy criteria.