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Just as important as the correct location of wildlife crossings is to have them properly designed to meet the performance objectives. Questions arise as to the size of the crossing and how species-specific behaviors should be incorporated into the crossing structure design. These concerns are offset by the logistics of the project, which include costs of the structure, available material and expertise, and physical limitations of the site, e.g., soil, terrain, hydrology. Stakeholders involved in the crossing structure design process can then find themselves searching through published and grey literature regarding the design, performance and cost of the project. As project managers attempt to incorporate the designs and lessons from other experiences, several general questions arise:
The general questions are followed by many specific questions:
This chapter provides examples of what tools and practical applications are available today for designing wildlife crossings in transportation projects. It is not meant to be a complete list of technical designs or methods used, but describe the most common wildlife crossing structure design types that are currently in use.
Wildlife crossing mitigation has two main objectives: 1) to connect habitats and wildlife populations and 2) reduce mortality of wildlife on roads as the Figure 18 chart shows.
Objective | Objective | |||
---|---|---|---|---|
Facilitate connections between habitats and wildlife populations | improve motorist safety and reduce wildlife-vehicle collisions 1 | |||
Wildlife Overpasses | Wildlife Underpasses | Specific Measures | Habitat Adaptation | Infrastructure Adaptation |
Landscape bridge (Sheet 1) Wildlife overpass (Sheet 2) Multi-use overpass {Sheet 3) Canopy crossing (Sheet 4) |
Viaduct or flyover (Sheet 5) Large mammal underpass (Sheet 6) Multi-use underpass (Sheet 7) Underpass with Waterflow (Sheet 8) Small to medium-sized mammal underpass (Sheet 9) Modified culvert (Sheet 10) Herpetile tunnel (Sheet 11) |
Fencing - Large mammals (Sheet 12) Fencing - small and medium vertebrates (Sheet 13) Gates and escape ramps (Sheet 14) Signage Animal - vehicle detection systems Speed reduction Lighting Reflectors |
Managing habitat and right-of-ways Intercept feeding |
Adapting road infrastructure (curbs, drainage, grates Jersey barriers) for wildlife movement Increasing width of road median |
Objective 1: Facilitate connections between habitats and wildlife populations
To achieve this goal, wildlife crossing structures are designed to allow movement of wildlife above or below road, either exclusively for wildlife use, mixed wildlife - human use, or as part of other infrastructure, e.g., creeks, canals. Wildlife crossing structures come in a variety of shapes and sizes, depending on their specific objective, and can be divided into 11 different design types (see Appendix C, Hot Sheets 1-11).
Objective 2: Improve motorist safety and reduce wildlife - vehicle collisions
Traffic-related mortality of wildlife can significantly impact some wildlife populations; particularly those that are found in low densities, slow reproducing, and need to travel over large areas. Common and abundant species like Deer, Elk and Moose can present serious problems for motorist safety. Many mitigation measures have been designed over the years to reduce collisions with wildlife; but few actually perform well or have been rigorously tested. Mitigation measures can be categorized as three types:
Objectives 1 and 2 should work together and can be integrated to provide for safe movements of wildlife across road corridors, by reducing motor vehicle accidents with wildlife. Wildlife crossings generally require one or more types of specific measures designed to improve motorist safety and reduce wildlife - vehicle collisions, e.g., fencing, escape gates and ramps (see Appendix C, Hot Sheets 12-14). Other techniques used to increase motorist safety and reduce collisions with wildlife, such as specific measures (signage and animal detection system) and the adaptation of habitats and road infrastructure, are not within the scope of this work. Detailed descriptions and guidelines for using these types of mitigation measures for wildlife can be found in Huijser et al. (2007a,b) and Iuell (2005).
Landscape connectivity is the degree to which the landscape facilitates wildlife movement and other ecological flows. However, no two landscapes are the same. Terrain, habitat type, levels of human activity and climate are some factors that influence wildlife movements and ecological flows. Therefore the spacing of wildlife crossings on a given section of roadway will depend largely on the variability of landscape, terrain, population densities, the juxtaposition of critical wildlife habitat that intersects the roadway and the connectivity requirements for different species. In landscapes that are highly fragmented with little natural habitat bisected by roadways shown in Figure 19, generally fewer wildlife crossings will be required compared to relatively intact, less fragmented landscapes as Figure 20 shows.
Wildlife crossings are permanent structures embedded within a dynamic landscape. With the lifespan of wildlife crossing structures around 70 - 80 years, the location and design of the crossings need to accommodate the changing dynamics of habitat and climatic conditions and their wildlife populations over time. How can we reconcile the dynamic environmental processes of nature with static physical structures on roadways? Environmental change is inevitable and will occur during the lifespan of the crossing structures. Some basic principles that management needs to consider:
These basic principles will help guide the determination of how many wildlife crossings may be necessary and how to locate them in order to get the greatest long-term conservation value. There is no simple formula to determine the recommended distance between wildlife crossings, as mentioned earlier each site is different. Planning will largely be landscape- and species- specific.
The spacing interval of some wildlife crossing projects designed for large mammals are found in Table 2. Listed are several large-scale mitigation projects in North America (existing and planned). The spacing interval varies from one wildlife crossing per 0.9 mi (1.5 km) to one crossing per 3.8 miles (6.0 km). The projects listed indicate that wildlife crossings are variably spaced but on average about 1.2 mi (1.9 km) apart.
Number of crossings | Road length (km) | Average Spacing/mile (km) | Location (Reference) |
---|---|---|---|
17 | 17 (27) |
1 / 1.0 (1 / 1.6) |
SR 260, Arizona USA (Dodd et al. 2007) |
24 | 27 (45) |
1 / 1.2 (1 /1.9) |
Trans-Canada Highway,a Banff, Alberta Canada (Clevenger et al. 2002) |
8 | 7.5 (12) |
1 / 0.9 (1 / 1.5) |
Trans-Canada Highway,b Banff, Alberta Canada (Parks Canada, unpubl. data) |
32 | 32 (51) |
1 /1.0 (1 / 1.6) |
Interstate 75, Florida USA (Foster and Humphries 1995) |
42 | 56 (90) |
1 / 1.3c (1 / 2.14) |
US 93, Montana USA (Marshik et al. 2001) |
16 | 15 (24) |
1 / 0.9 (1 / 1.5) |
Interstate 90, Washington USA (Wagner 2005) |
4 | 15 (24) |
1 / 3.8 (1 / 6.0) |
US 93 Arizona USA (McKinney and Smith 2007) |
82 | 45 (72) |
1 / 0.5 c (1 / 0.9) |
A-52, Zamora Spain (Mata et al. 2005) |
Earlier, the 11 different wildlife crossing design types were introduced. Their intended use and function are each described below.
Determining the type of wildlife crossing structure most suitable for a given location will depend on several criteria. Selection begins by identifying a general wildlife crossing type that conforms to the wildlife habitat connectivity potential for the target species and topography of the site chosen. Figures 21, 22 and 23 can be used to guide the selection of wildlife crossing type based on the two main criteria - quality of wildlife habitat and topographical constraints.
Wildlife habitat connectivity potential can be grouped into three categories:
Topography strongly influences what type of wildlife crossing can be built at each location. The proximity to water (lakes, ponds, rivers, streams) is another factor, as is the water table at the location, but these factors will not be discussed here. Four general topographies have been identified where wildlife crossings may be constructed on roadways as sketched in Figure 24.
Planning and designing wildlife crossings will often be focused on a certain species of conservation interest (e.g., threatened or endangered species), a specific species group (e.g., amphibians) or abundant species that pose a threat to motorist safety (e.g., Deer, Elk).
In this handbook we refer to North American wildlife and species groups when discussing the appropriate wildlife crossing designs. The eight groups mentioned below are general in composition. However, recommendations will be provided, if it is available, for species-specific design requirements (Appendix C, Hot Sheets 1-11). Their ecological requirements and how roads affect them are described along with some sample wildlife species for each group.
Type | Usage | Species & Groups | Dimensions Minimum | Dimensions Recommended |
---|---|---|---|---|
Landscape bridge | Wildlife only | All wildlife species Amphibians (if adapted) |
W: 230 ft (70 m) |
W: >330 ft (>100 m) |
Wildlife overpass | Wildlife only | Large mammals High-mobility medium-sized mammals Low mobility medium-sized mammals Small mammals Reptiles Amphibians (if adapted) |
W: 130 - 165 ft (40 - 50 m) |
W: 165 - 230 ft (50 - 70 m) |
Multi-use overpass | Mixed use: Wildlife & Human activities | Large mammals High-mobility medium-sized mammals Low mobility medium-sized mammals Small mammals Amphibians (if adapted) Reptiles |
W: 32 ft (10 m) |
W: 50 - 130 ft (15 - 40 m) |
Canopy crossing |
Wildlife only | Semi-arboreal mammals |
- | - |
Type | Usage | Species groups | Dimensions: Minimum | Dimensions: Recommended |
---|---|---|---|---|
Viaduct or flyover | Multi- purpose | All wildlife species | There are no minimum dimensions. Structures are generally larger than the largest wildlife underpass structures | There are no recommended dimensions. Structures are generally larger than the largest wildlife underpass structures |
Large mammal underpass | Wildlife only | Large mammals High-mobility medium- sized mammals Low mobility medium- sized mammals Semi-arboreal & semi- aquatic mammals (adapted) Small mammals Amphibians (adapted) Reptiles |
W: 23 ft (7 m) Ht: 13 ft (4 m) |
W: >32 ft (>10 m) Ht: >13 ft (>4 m) |
Multi-use underpass | Mixed use: Wildlife & Human activities | Large mammals High-mobility medium- sized mammals Low mobility medium- sized mammals Semi-arboreal & semi- aquatic mammals (adapted) Small mammals Amphibians (adapted) Reptiles |
W: 16.5 ft (5 m) Ht: 8.2 ft (2.5 m) |
W: >23 ft (>7 m) Ht: >11.5 ft (>3.5 m) |
Underpass with waterflow | Wildlife and drainage | Large mammals High-mobility medium- sized mammals Low mobility medium- sized mammals Semi-arboreal mammals (adapted) Semi-aquatic mammals Small mammals & amphibians Semi-arboreal mammals & reptiles (adapted) |
W*: 6.5 ft path (2 m) Ht: 10 ft (3 m) *Width will be dependent on width of hydrologic channel in crossing |
W*: >10 ft path (>3 m) Ht: >13 ft (>4 m) *Width will be dependent on width of hydrologic channel in crossing |
Small to medium-sized mammal underpass | Wildlife and seasonal drainage | High-mobility medium- sized mammals (adapted) Low mobility medium- sized mammals Semi-aquatic mammals (adapted) Small mammals Amphibians (adapted) Reptiles |
Same as recommended dimensions Size selection is based on the target species needs or connectivity objective at the site. | W: 1-4 ft (0.3 - 1.2 m) Ht: 1-4 ft (0.3 - 1.2 m) OR 1 - 4 ft diameter (0.3 - 1.2 m) |
Modified culvert | Wildlife and drainage | High-mobility medium- sized mammals (adapted) Low mobility medium- sized mammals Semi-aquatic mammals Small mammals Reptiles (adapted) Amphibians |
W: 1.5 ft (0.5 m) Clearance: >3 ft (>1 m) |
W: >3 ft (>1 m) Clearance: >4 ft (>1.5 m) |
Amphibian and reptile tunnel | Wildlife only | Amphibians Low mobility medium- sized mammals (adapted) Semi-aquatic (adapted) Small mammals & reptiles (adapted) |
Dimensions vary depending on target species or taxa or local conditions. Tunnels range from 1 - 3 ft (0.35 - 1 m) in diameter |
Dimensions vary depending on target species or taxa or local conditions. Tunnels range from 1 - 3 ft (0.35 - 1 m) in diameter |
The dimensions shown earlier in Tables 3 and 4 are meant to serve as a general guideline when planning and designing for species groups or taxa. However, oftentimes project objectives are species-specific and design must be customized to their needs.
Our monitoring and research of crossing structures in North American during the last 10 years has yielded valuable information on design needs of a variety of wildlife species. Research results were published in scientific journals and internal agency reports. In Table 5 we synthesized the research results to determine the suitability of the 11 crossing structure types for the most common wildlife species or taxonomic groups in North America. We list 26 wildlife species or taxa and we categorize the suitability of each of the 11 crossing design types for each species as follows:
Landscape bridge (Sheet 1) | Wildlife overpass (Sheet 2) | Multi- use overpass (Sheet 3) | Canopy crossing (Sheet 4) | Viaduct or flyover (Sheet 5) | Large mammal underpass (Sheet 6) | Multi-use underpass (Sheet 7) | Underpass with waterflow (Sheet 8) | Small- to medium- sized mammal underpass (Sheet 9) | Modified culvert design (Sheet 10) | Amphibian and reptile tunnel (Sheet 11) | |
---|---|---|---|---|---|---|---|---|---|---|---|
Ungulates | |||||||||||
Moose | ● | ● | ⦻ | - | ● | ◎ | ⦻ | ◎ | ⦻ | ⦻ | - |
Elk | ● | ● | ● | - | ● | ● | ● | ● | ⦻ | ⦻ | - |
Deer sp. | ● | ● | ● | - | ● | ● | ● | ● | ⦻ | ⦻ | - |
Pronghorn | ● | ● | ⦻ | - | ● | ? | ⦻ | ◎ | ⦻ | ⦻ | - |
Bighorn sheep | ● | ● | ⦻ | - | ● | ◎ | ⦻ | ◎ | ⦻ | ⦻ | - |
Mountain goat | ● | ● | ⦻ | - | ● | ◎ | ⦻ | ◎ | ⦻ | ⦻ | - |
Carnivores | |||||||||||
Black bear | ● | ● | ⦻ | - | ● | ● | ⦻ | ● | ⦻ | ⦻ | - |
Grizzly bear | ● | ● | ⦻ | - | ● | ◎ | ⦻ | ◎ | ⦻ | ⦻ | - |
Wolf | ● | ● | ⦻ | - | ● | ◎ | ⦻ | ◎ | ⦻ | ⦻ | - |
Coyote | ● | ● | ● | - | ● | ● | ● | ● | ● | ◎ | - |
Fox1 (V vulpes, Urocyon) | ● | ● | ● | - | ● | ● | ● | ● | ● | ◎ | - |
Fox2 (V macrotis, V velox) |
● | ● | ● | - | ● | ◎ | ◎ | ◎ | ◎ | ◎ | - |
Cougar | ● | ● | ⦻ | - | ● | ● | ⦻ | ● | ⦻ | ⦻ | - |
Bobcat | ● | ● | ● | - | ● | ● | ● | ● | ⦻ | ◎ | - |
Lynx | ● | ● | ⦻ | - | ● | ? | ⦻ | ? | ⦻ | ⦻ | - |
Wolverine | ● | ● | ⦻ | - | ● | ? | ⦻ | ? | ⦻ | ⦻ | - |
Fisher | ● | ● | ● | ● | ● | ◎ | ● | ◎ | ● | ● | - |
Marten | ● | ● | ● | ● | ● | ◎ | ● | ◎ | ● | ● | - |
Weasel | ● | ● | ● | - | ● | ◎ | ● | ◎ | ● | ● | - |
Badger | ● | ● | ● | - | ● | ◎ | ● | ◎ | ● | ⦻ | - |
Low mobility medium | ● | ● | ● | - | ● | ● | ● | ● | ● | ● | ◎ |
---|---|---|---|---|---|---|---|---|---|---|---|
Semi-arboreal mammals | ◎ | ◎ | ◎ | ● | ◎ | ◎ | ◎ | ◎ | ⦻ | ⦻ | ⦻ |
Semi-aquatic mammals | ◎ | ◎ | ◎ | - | ◎ | ◎ | ◎ | ● | ◎ | ● | ◎ |
Small mammals | ● | ● | ● | ◎ | ● | ● | ● | ● | ● | ● | ◎ |
Amphibians | ◎ | ◎ | ◎ | - | ◎ | ◎ | ◎ | ◎ | ◎ | ● | ● |
Reptiles | ● | ● | ● | - | ● | ● | ● | ◎ | ● | ● | ◎ |
Detailed design information for the 26 species and 11 crossing structure types are found in Appendix C, Hot Sheets 1-11.
The Hot Sheets are a guide for the general design, basic building prescriptions, landscaping, possible design variations, and maintenance of each of the 11 crossing structure types. Being a logical endpoint for this chapter, by starting broadly and progressively narrowing the taxonomic focus, the Hot Sheets provide the most detailed design guidelines for the 26 wildlife species and taxa in North America.
Fencing is a key part of a mitigation plan involving wildlife crossings. Hot Sheets 12-14 provide details on fence configurations, construction specifics, design alternatives and maintenance.
Fences and wildlife crossings have been around many years, however, relatively little is known about effective fence designs and other innovative solutions to keep wildlife away from roads and traffic.
Small- and medium-sized mammals can pass through most fence types for large mammals. Different fencing types and designs are needed to keep these smaller animals from reaching roads (Hot Sheet 13).
When wildlife become trapped inside fenced areas measures need to be in place to allow them to safely exit the right-of-way. Steel swing gates, hinged metal doors or earthen ramps or jump-outs are some commonly used methods (Hot Sheet 14).
Bissonette, J.A. 2007. Evaluation of the use and effectiveness of wildlife crossings. National Cooperative Highway Research Program (NCHRP) 25-27 final report. Transportation Research Board, Washington DC.
Clevenger, A.P. & Waltho, N. 2000 Factors influencing the effectiveness of wildlife underpasses in Banff National Park, Alberta, Canada. Conservation Biology, 14, 47- 56.
Clevenger, A.P. & N. Waltho. 2005. Performance indices to identify attributes of highway crossing structures facilitating movement of large mammals. Biological Conservation 121:453-464.
Dodd, C.K., W.J. Barichivich, and L.L. Smith. 2004. Effectiveness of a barrier wall and culverts in reducing wildlife mortality on a heavily traveled highway in Florida. Biological Conservation 118, 619-631.
Dodd, N., J. Gagnon, S. Boe, A. Manzo and R. Schweinsburg. 2007. Evaluation of measures to minimize wildlife - vehicle collisions and maintain permeability across highways: Arizona Route 260. Final report prepared for Arizona Department of Transportation, Phoenix, Arizona.
Forman, R.T.T., Sperling, D., Bissonette, J., Clevenger, A., Cutshall, C., Dale, V., Fahrig, L., France, R., Goldman, C., Heanue, K., Jones, J., Swanson, F., Turrentine, T. & Winter, T. 2003. Road ecology: Science and solutions. Island Press, Washington, D.C.
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Iuell, B. (ed.). 2005. Wildlife and traffic: A European handbook for identifying conflicts and designing solutions. KNNV Publishers, Utrecht, The Netherlands.
Langton, T.E.S. (Ed.). 1989. Amphibians and roads. ACO Polymer Products Ltd., Bedfordshire, England. 199 pages.
Lebarrères, D., T. Lodé, and J. Merilä. 2003. What type of amphibian tunnel could reduce road kills? Oryx 38, 220-223.
MacKenzie, D.I., J.D. Nichols, J.A. Royale, K.H. Pollock, L.L. Bailey, J.E. Hines. 2006. Occupancy estimation and modeling: Inferring patterns and dynamics of species occurrence. Academic Press, New York, NY.
Ministerio de Medio Ambiente. 2006. Prescripciones técnicas para el diseño de pasos de fauna y vallados perimetrales. Documentos para la reducción de la fragmentación de hábitats causada por infraestructura de transporte. Parques Nacionales. Ministerio de Medio Ambiente. Madrid, Spain. 112pp.