Most studies and publications about sizing wildlife structures have a narrow focus and leave out actual variables, such as allowable lengths, widths, and heights for acceptable success rates for the passage of a target species. In 2011, FHWA published the Wildlife Crossing Structure Handbook Design and Evaluation in North America (FHWA-CFL/TD-11-003) to provide guidance. This handbook created “hot sheets” for different categories of wildlife structures, providing some general guidance. However, the science around providing safe passages through our highways for wildlife has evolved since then.

There is currently no single clearinghouse of recommendations or criteria that is recognized nationwide. Standard practice in providing permeability and connectivity along highways can vary from state to state and even project to project within each state. Some states have minimal experience with the placement and sizing of safe wildlife passages and could learn from other states’ experiences. Engineering staff, as part of a multidisciplinary team, look for criteria and guidance when implementing any design. A synthesis of best practices across the nation could supplement or enhance this effort and help to guide states with useful variables for successful implementation of their own wildlife structures. 

The research objective is to take stock of dimensions, variables, and best practices when it comes to the design and layout of wildlife passage structures across highways. Having a commonality or shared knowledge of successful practice of the sizing and placing of wildlife passage structures over or under highways can help other states have success in their early endeavors or help other more experienced states fine-tune their practices.