Permeability of Existing Structures for Wildlife : A Passage Assessment System

PDFTech ReportWA-RD 777.1Wildlife crossingsBridges and culvertsWashingtonWashington (State). Dept. of Transportation. Office of Research and Library ServicesKintsch, JuliaCramer, PatriciaWashington (State). Dept. of Transportation. Office of Research and Library ServicesUnited States. Federal Highway AdministrationUS Transportation Collection"A Passage Assessment System (PAS) was developed to help the Washington Department ofTransportation (WSDOT) evaluate existing transportation infrastructure for its ability to facilitateterrestrial wildlife movement from one side of a roadway to the other. The outcomes of thisresearch provide mechanisms to allow transportation agencies to identify both opportunities andbarriers to wildlife passage along roads. The PAS presented in this report provides an assessmentprocess that differentiates \ue2\u20ac\u201c for different types of wildlife \ue2\u20ac\u201c between structures that are currentlyfunctional, 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 Classeswere also defined to classify road infrastructure, such as bridges, culverts and pipes, to create acommon understanding of terminology related to wildlife crossings. The field research wasconducted for six months in six locations in central and western Washington with the objective ofascertaining which species of wildlife approached and used different types of structures. The PASwas then developed on the basis of these field data, research on wildlife use of crossings acrossNorth America, and the organization provided by the Species Movement Guilds and StructuralFunctional Classes. The PAS guides practitioners through a series of targeted questions designedto characterize a bridge or culvert relative to its potential to function as a wildlife passage for thefull range of wildlife known to occur at a given site. The PAS is intended as an evaluation tool toensure that biologists ask the right questions in the field and fully document the conditions thatmay affect passage functionality for the diversity of target species. By answering questions aboutthe structure characteristics, vegetation, land use, roadway, barriers and fencing, the biologist willhave 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 providesan effective mechanism for determining which structures are suitable for enhancements toimprove their functionality as wildlife passages or, if no such enhancements are appropriate,identify structure replacement needs for improved highway permeability for wildlife. A PassageEnhancement toolbox is provided to complement the PAS and presents a number of infrastructureadjustments and maintenance actions that may be implemented to help wildlife better movethrough structures. This toolbox may be used to help guide users in developing site-specificrecommendations.The Passage Assessment System supports timely inclusion of wildlife passage needs from theonset of highway corridor planning, project planning and design. It offers potential cost-savingsand minimized project delays by identifying passage modifications that may be significantly lesscostly 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 an