Permeability of Existing Structures for Terrestrial Wildlife: Developing a Passage Assessment System

A Passage Assessment System (PAS) was developed to help the Washington State Department of Transportation (WSDOT) evaluate existing transportation infrastructure for its ability to facilitate terrestrial wildlife movement from one side of a roadway to the other. The outcomes of this research provide mechanisms to allow transportation agencies to identify both opportunities and barriers to wildlife passage along roads.

The PAS presented in this report provides a wildlife passage assessment process that differentiates – for different types of wildlife – between structures that are currently functional, those that could be enhanced to become more functional, and those that are not, based on how the wildlife respond to roads and crossing structures. Structural Functional Classes were also defined to classify road infrastructure, such as bridges, culverts and pipes, to create a common understanding of terminology related to wildlife crossings.

The field research was conducted for six months in six locations in central and western Washington with the objective of ascertaining which species of wildlife approached and used different types of structures. The PAS was then developed on the basis of these field data, research on wildlife use of crossings across North America, and the organization provided by the Species Movement Guilds and Structural Functional Classes. The PAS guides practitioners through a series of targeted questions designed to characterize a bridge or culvert relative to its potential to function as a wildlife passage for the full range of wildlife known to occur at a given site.

The PAS is intended as an evaluation tool to ensure that biologists ask the right questions in the field and fully document the conditions that may affect passage functionality for the diversity of target species. By answering questions about the structure characteristics, vegetation, land use, roadway, barriers and fencing, the biologist will have a complete passage assessment including preliminary ideas for improving the structure, which can be further refined during the project planning and design processes. The PAS provides an effective mechanism for determining which structures are suitable for enhancements to improve their functionality as wildlife passages or, if no such enhancements are appropriate, identify structure replacement needs for improved highway permeability for wildlife.

A Passage Enhancement toolbox is provided to complement the PAS and presents a number of infrastructure adjustments and maintenance actions that may be implemented to help wildlife better move through structures. This toolbox may be used to help guide users in developing site-specific recommendations.

The Passage Assessment System supports timely inclusion of wildlife passage needs from the onset of highway corridor planning, project planning and design. It offers potential cost-savings and minimized project delays by identifying passage modifications that may be significantly less costly than new infrastructure. Where existing culverts and bridges can be shown to pass wildlife, it would help to reduce future construction costs for wildlife crossings in those areas and help to prioritize which areas are lacking in potential crossings and need additional mitigation.

Publication Date: 
Friday, July 1, 2011
Publication Number: 
WA-RD 777.1
Last modified: 
01/17/2019 - 11:52
Julia Kintsch, Patricia Cramer.
Utah State University. Dept. of Wildland Resources.; ECO-resolutions, LLC
Number of Pages: 
Wildlife crossings, Evaluation and assessment, Bridges and culverts, Animal migrations, Habitat, Roadside fauna, Highway facilities for nonmotorized users, Wildlife, Retrofitting, Animal behavior.