NCHRP Project 25-27, Evaluation of the Use and Effectiveness of Wildlife Crossings, will explore methods used by state transportation agencies in tracking and funding maintenance needs, tracking wildlife-vehicle collisions, and the extent to which such information is eventually used in identifying sites for mitigation measures. One of the best sources of existing information is NCHRP Synthesis 305, Interaction between Roadways and Wildlife Ecology: a Synthesis of Highway Practice, which reviews a number of opportunities and best practices related to maintenance of structures. These are excerpted as follows: [N]

Bats and Birds in Bridges
An emerging area for maintenance related to wildlife concerns bats in bridges. Keeley and Tuttle (1996) describe the use of highway bridges and culverts as bat habitats and provide guidance for maintenance and demolition of bridges occupied by bats. They report that some states, such as Texas, are managing bridges for bats with great success. Washington State DOT has developed tracking programs for birds in bridges and maintenance inspection personnel. Maintenance personnel must be aware that some species of bats and birds are listed as threatened or endangered. It is usually necessary to bring in environmental professionals when bats and birds are founds. Culverts
Materials used in modern culvert construction (concrete and metal with protective coatings) and the actual design (corrugated) can result in a structure with a long life span and potentially little maintenance. Several states have developed manuals to address the problems associated with culvert maintenance. A common problem with the maintenance of ordinarily dry culverts in upland areas is the control of vegetation in keeping the structure open and accessible. Deposition soil around the mouth of small pipe culverts as a result of wind and rain can result in decreased effectiveness for wildlife movement.

Underpasses
Because wildlife underpasses are essentially bridges over land and water, maintenance personnel can expect routine structural inspection and maintenance activities as for any bridge structure. Slope maintenance around these crossings is often problematic because of the need to maintain a built-up fill section for an elevation that provides for a smooth transition into the bridge while maintaining suitable conditions for animal movement under the bridge. Slope stabilization with headwalls, riprap, reinforced earth, or vegetation can greatly reduce maintenance frequency, expense, and disturbance to the wildlife underpass. Many underpasses are large enough that maintenance of the cross-sectional opening is not as problematic as it can be in some drainage culverts. It is important that cover for animals be a consideration in the maintenance plan for the structure. If organisms sensitive to the need for cover are to use the structure, maintenance of sufficient cover will be required. Research from Europe has indicated that cover, such as rows of debris under the crossing, can facilitate small mammal and reptile/amphibian movement under the crossing.

To assure visibility of the crossings for animals, vegetation control is the primary maintenance function for these structures. Therefore, it may be necessary to size structures so that mowers can move through the underpass and the area in and around the structure. Graffiti and vandalism are also maintenance problems in areas that have access to humans.

Overpasses
Overpasses for wildlife are so recent in the United States that good information about their maintenance is not available. In Europe, maintenance on overpasses is performed for native vegetation and even wetland systems, similar to that for adjacent roadside communities. Various structures for wildlife cover, including large rocks and stumps, are maintained on European overpasses. With the exception of planting and maintenance of native vegetation, Europeans do little else to maintain their wildlife overpasses. In Canada, one innovative measure being used in Banff National Park involves the placement of piles of used Christmas trees to provide cover for habitat and movement of small animals across the overpasses. Fencing
Fence maintenance can be one of the most expensive activities for wildlife mitigation techniques. Run-off-the-road vehicles and falling trees often damage fences and unless quickly repaired animals will find their way through these breeches and on to the rights-of-way. Vegetative growth along fences can also present a maintenance problem. Spraying with herbicides seems to be the most popular maintenance measure, although this can present problems in particularly sensitive aquatic areas and areas with listed protected plants.