The National Academies

NCHRP 25-25/Task 113 [Pending]

Highway Passages for Small Terrestrial Wildlife - Summary and Repository of Design Examples
[ NCHRP 25-25 (Research for the AASHTO Committee on Environment and Sustainability) ]

  Project Data
Funds: $125,000
Contract Time: 18 months
Staff Responsibility: Ann M. Hartell

There is a growing need to better understand the functionality of drainage and other passage structures to facilitate safe passage of smaller wildlife in terrestrial habitats via retrofits, rehabilitation, or minor additions to planned transportation projects as a result of increased listing of species under the Endangered Species Act and a decrease in funding availability for large highway projects.

The relationship between wildlife connectivity and roads has a robust history of research; however, much of this research has focused on large and migratory wildlife (e.g. deer, elk and bears) because of safety concerns and the costs associated with wildlife-vehicle collisions, as well as availability of recorded roadkill data. Several NCHRP studies have been conducted mainly focused on larger wildlife. These include NCHRP Synthesis 305, NCHRP Report 615 and NCHRP 25-25 Tasks 68, 84 and 93. Recent academic research volumes have also been published discussing general design considerations for animals of all sizes, but do not contain specific information to assist DOTs in developing designs and comparing costs for specific project implementation.

Since the Nutty Narrows squirrel bridge was constructed in Longview, Washington in 1963, a wide range of projects have been implemented by state DOTs, local governments, and non-governmental organizations to remedy situations with high levels of road-related mortality for smaller wildlife,  reptiles and amphibians. These projects include incorporation of measures into new roads, retrofits of existing infrastructure, and addition of infrastructure specifically for wildlife use and many also implement post-construction monitoring of various types. However, many of these projects are not known beyond the local area or state where they were constructed. As a result, the designs and lessons learned from their implementation are not available to inform current projects. 

This research is particularly timely because many smaller wildlife species are being reviewed for listing under the Endangered Species Act as a result of various legal settlements. Information on solutions to avoid traffic-related mortality and the loss of landscape connectivity for these species will assist DOTs with addressing larger ecosystem-level effects of road projects, as recently proposed by the US Fish and Wildlife Service. In addition, this information will be helpful for integrating wildlife accommodations into climate resilience-related culvert retrofit projects. 


The purpose of this study is to (1) gather and summarize existing information on existing crossing structures and methods used by small terrestrial mammals, reptiles and amphibians, and (2) develop a central repository of plan sheets and case studies of successes and lessons learned for a representative selection of projects.

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