Best Practices Manual: Wildlife Vehicle Collision Reduction Study

CHAPTER 3: CORRIDOR PLANNING AND DESIGN

While chapter 2 focused on the regional or statewide scale, this chapter focuses on mitigation at the corridor scale, such as a single reconstruction project (figure 5). Many of the principles applied at the regional scale are applicable to the corridor scale, but involve a different implementation.

WVCs should be considered at every step in the highway planning and design process. In order to guide the mitigation of WVCs at the corridor level:

• Target species and concerns (safety, conservation, or both) should first be identified (section 3.1). These species could include threatened or endangered species present along the corridor, or the most common large animal species involved in WVCs in the area.
• Consideration should be given to the location and alignment of the roadway to avoid potential problem areas that can sometimes avoid WVC issues (section 3.2).
• In addition to alignment, roadway design may include principles that reduce the frequency of WVCs (section 3.3).
• Existing and potential animal crossing locations should be identified (section 3.4) in order to incorporate mitigations in the highway design.
• Instead of mitigating at all (potential) crossing locations, wildlife may be funneled to a selection of these locations where crossing opportunities are provided. Section 3.5 provides guidelines concerning optimal distances between crossing opportunities.

Figure 5. Corridor planning and design as part of a strategy for reducing WVCs.

3.1 STEP 1: TARGET SPECIES

Species differ in their habitat requirements and thus have different movement patterns. It is important to identify the target species early in the design process and to understand the movement patterns that need to be considered. Three types of animal movement are most commonly considered (figure 6):

• The home range is an area within which an animal typically remains to conduct daily activities. Typical movements within the home range include foraging (figure 6A) and diurnal movements (figure 6B). The size of the home range depends on the species, which then will have an impact on the design of the spacing between wildlife road crossing mitigation measures.
• Dispersal refers to movements of wildlife to establish new home ranges outside of their current home range or their natal (birth) range. Essentially, dispersal is the process where individuals or populations change the area they occupy on a permanent basis. Individual animals may seek new territories or ranges due to population density-dependent competition, or limited resources within their existing range. Animals may also be forced to disperse by parents or family units, or expelled from their existing ranges by rival individuals through competition. At the population level, dispersal results in the expansion, redistribution, or contraction of an entire population or species distribution.
• Some species are migratory (figure 6D)-that is, they display seasonal movements between different areas. These movements can be very predictable in terms of their location and time of year.

Figure 6. Basic wildlife movements (reprinted with permission from Trocmé et al. 2003). ¹⁰

3.2 STEP 2: ROAD ALIGNMENT CONSIDERATIONS

The location or alignment of a roadway can have a substantial impact on the magnitude of WVCs that may occur after the road is constructed. Thus, integration of transportation and conservation planning at the initial design stages, when location/alignment is discussed, is essential to addressing WVCs.

With this integration, the route can be laid out with consideration of animal presence, animal movements, and ecological processes. Typically these efforts are aimed at conservation, but they can also substantially reduce WVCs. Several models are used to assess the ecological impact on wildlife of new roadway alignment options:

• Maurer developed one such model for the Pennsylvania Department of Transportation. ¹¹ A community-based, landscape-level terrestrial decision support system was developed with the objective of maintaining the ecological integrity of Pennsylvania ecosystems. Variables for Assessing Reasonable Mitigation in New Transportation (VARMINT) includes habitat importance, stewardship, patch size and shape, connectivity, proximal land use, relative significance, natural processes, diversity, anthropic use, and intangibles. Alternative routes are assigned scores for each of these criteria.comparison of scores determines relative ranking.
• Metroquest is a scenario-planning tool that incorporates expected development and transportation options, providing visualization for stakeholder and public involvement. It works throughout the planning cycle, and can be customized for a given region (http://www. questforthefuture.com).

Context sensitive design/context sensitive solution (CSD/CSS) processes are used to incorporate community values, aesthetics, and environmental and other priorities into the road design process. Using the CSD/CSS process could result in alignments that minimize the impact of WVCs. For more information on the overall CSD/CSS process, refer to the following web site and reports:

• FHWA (http://www.fhwa.dot.gov/context/what.cfm).
• NCHRP 480 A Guide to Best Practices for Achieving Context Sensitive Solutions. ¹²
• AASHTO Guide for Achieving Flexibility in Highway Design. ¹³
• NCHRP Report 69: Performance Measures for Context Sensitive Solutions-A Guidebook for State DOTs. ¹⁴ Note that this report details how to develop a CSD/CSS program and track performance with surveys. Although there is some discussion on developing performance criteria (at the project and programmatic level), specific measures are not discussed (despite the title).

3.3 STEP 3: ROAD DESIGN CONSIDERATIONS

Consideration of some basic WVC mitigation principles in designing various elements of a highway could minimize the potential for WVCs. Some of these are discussed in the mitigations chapters (4 and 5) but could also be implemented as part of the initial highway design. The considerations mentioned in this section should be taken into account when designing roadways that have a high likelihood for WVCs:

• Two-lane rural/suburban roads.
• Low- and medium-volume highways that pass through wildlife habitat.

Consideration 1: Steep Side Slopes

Slopes can hide approaching animals from a driver's view. The AASHTO Green Book recommends slopes of 1 m vertical to 1 m horizontal or flatter. ¹⁵ It further recommends that slope transitions be gently rounded. This geometry allows the driver a clear view of the area adjacent to the roadway (whether cut or fill). Designers should use steeper fill slopes with caution, as drivers may not be able to see animals approaching the roadway until the animal leaps over the guardrail. If a steeper fill slope is used:

Figure 7. Example of box beam guardrail (copyright: Marcel Huijser).

Consideration 2: Known or Anticipated Hotspots

If no data exist on hotspots (e. g. , for a new road), one can anticipate that problem locations will likely exist at drainage crossings, known migration corridors, or known animal habitat. Ideally such potential problem areas are avoided or minimized during the planning process and selection of the road alignment. However, should these potential problem areas still be present in the final alignment, the designer should take extra caution to make animals visible to drivers at these locations by:

Consideration 3: Culvert or Bridge Installations

Consideration 4: Roadside Ditches and Drainage

Consideration 5: Roadside Vegetation

Consideration 6: Median Barriers

Concrete median barriers (figure 8) can cause problems for wildlife. When crossing the roadway, wildlife may pause at the barrier or turn around, increasing their time in the roadway. A summary of the literature by Clevenger and Kociolek found that "the general hypothesis is that concrete Jersey barriers may increase the risk of direct vehicle mortality [of wildlife]. " ¹⁶ Mitigations include:

Figure 8. Long sections of median barriers are thought to increase road mortality and reduce animal movements across the road. Note that the small cutouts at the bottom of concrete median barrier are designed for drainage but also allow small animal species to cross under the median barriers (copyright: Marcel Huijser).

Figure 9. Concrete barriers with scuppers that allow small and mid-sized species to cross under the median barriers (copyright: A. P. Clevenger).

Figure 10. Opening in median barriers designed to allow wildlife to cross the road and the median barriers (copyright: Marcel Huijser).

In summary, the designer should estimate the magnitude of the potential WVC problem and include adaptations when designing a road. If there are areas along the route that have a high potential for WVCs, the designer should consider including mitigations mentioned in chapters 4 and 5.

3.4 STEP 4: IDENTIFY AND PRIORITIZE LOCATIONS WHERE WILDLIFE CROSS THE ROAD

Even if WVCs are considered in road alignment and design, problem locations may still be found following construction. For the target species and the road segment considered, WVC priority locations should be identified and mitigated (see chapters 4 and 5 for mitigation options). There are several data sources that can be used to identify where wildlife cross the road:

• Road mortality data might seem best suited for determining where wildlife crossings should be placed. However, research has shown that the locations where wildlife are struck by vehicles may have little in common with where they cross roads safely. ¹⁷ Motorist visibility, road width and curvature, and vegetative cover adjacent to the road are some of the factors that explain wildlife-vehicle collision patterns. These may have little bearing on where safe road crossings occur. Road mortality data are good for identifying WVC hotspots, but should not be used as the sole source for the location of wildlife crossing opportunities. Consulting additional sources on wildlife movements (e. g. , habitat linkage mapping or movement models) is required to avoid blocking animal movements at locations where wildlife may cross the road successfully and where no or little road mortality is recorded.
• Radio-telemetry has been commonly used to identify successful road crossing locations, usually through intensive monitoring of movements of individual animals. Accurate and abundant location data are typically obtained through the use of GPS collars. ¹⁸
• Roadside surveys for tracks can be used in areas that receive regular snowfall. Animal tracks can be identified and recorded while driving slowly along the road edge. ¹⁹
• Roadside surveys for direct animal sightings. ²⁰
• GIS-based wildlife movement or population viability modeling (see also chapter 2).
• Use local experts (expert-based GIS modeling, rapid assessment) and knowledge of target species movement if data from above methods are unavailable (see also chapter 2).
• Locations where streams or rivers cross the road, or where edge habitat and cover comes close to the road are often associated with areas of frequent wildlife crossing.

3.5 STEP 5: DETERMINE SPACING OF CROSSING IF NEEDED

If the WVC mitigation measure itself causes a barrier to wildlife movement (i. e. fencing), animal crossing structures may be needed. These wildlife crossings opportunities should be located at identified sites where wildlife cross the road. This section provides guidelines on the spacing of these crossing opportunities based on the size of their home range.

When wildlife fencing is installed alongside a road, the barrier effect of the road corridor is increased. Depending on the species concerned, a wildlife fence may be an absolute or a nearly complete barrier. Such barriers in the landscape are to be avoided as they isolate animal populations, and smaller and more isolated populations have reduced population survival probability. Therefore, when a wildlife fence is installed, safe crossing opportunities for wildlife should be provided. This section discusses optimal distances between safe crossing opportunities.

The spacing of safe crossing opportunities for wildlife can be calculated in more than one way and is dependent on the goals one may have. Examples of possible goals are:

• Provide permeability under or over the road for ecosystem processes, including but not restricted to animal movements. Ecosystem processes include not only biological processes, but also physical processes like water flow.
• Allowing multiple species to change their spatial distribution through dispersal or migration.
• Maintaining or improving the population viability of selected species based on their current spatial distribution. This includes striving for larger populations with a certain degree of connectivity between populations (allowing for successful dispersal movements).
• Providing the opportunity for individuals (and populations) to continue seasonal migration movements, like those seen among big horn sheep and mule deer.
• Allowing access to the targeted species' habitat on both sides of a roadway when a road traverses their home range. This may result in a road corridor that is, at least partially, permeable for wildlife particularly for individuals residing near the roadway.

While population viability analyses can be very helpful in comparing the effectiveness of different configurations of safe crossing opportunities, the data required for such analyses are often unavailable or incomplete. Furthermore, the collection of such data is typically very time consuming and can be expensive, especially if multiple species are to be investigated. For this handbook the authors describe two alternative approaches:

• Base the spacing of safe crossing opportunities on current practices on other mitigated road sections.
• Base the spacing of safe crossing opportunities on the diameter of the home range sizes of the target species.

Current Practice

Large mammal crossing structure spacing at eight different road sections in the United States and Canada are listed in Table 1.The average spacing for the large mammal crossing structures listed one per 2.1 km (1.3 mi). Note that this spacing does not necessarily have a biological basis, it simply provides a reference on the current practice for the spacing of these large-mammal crossing opportunities.

Table 1. Spacing interval of wildlife crossings for large mammals at existing and future road sections with crossing structures for wildlife.

^aIncludes crossing for small and large mammals.

Spacing Based on Home Range Sizes

Another approach is to base the distance between safe crossing opportunities on the diameter of the home range of individual species. Estimates of the home range size for various species are presented in appendix A. Using this approach, the distance between safe crossing opportunities is simply set to the diameter of the home range of the species concerned (figure 11). This approach allows access to at least one safe crossing opportunity for individuals that have the center of their home range on the road (such as individuals "x" or "z" in figure 12). However, individuals that have their home range on both sides of the road do not necessarily have access to a safe crossing opportunity (such as individual "y" in figure 12). Finally, this approach assumes homogenous habitat and distribution of the individuals and circular home ranges, while in reality habitat and habitat quality may vary greatly, causing variations in density of individuals and irregularly shaped home ranges.

It should be emphasized that this approach may not necessarily maintain viable populations for every targeted species, and that not every individual that approaches the road and associated wildlife fence will encounter and use a safe crossing opportunity. In addition, the approach described above is not necessarily the only approach or the approach that addresses the barrier effect of the road corridor and associated fencing sufficiently for all species concerned. However, this approach:

• Treats different target species consistently, based on the available data (or lack thereof).
• Seems practical.
• Is likely to result in considerable permeability of the road corridor and associated wildlife fencing for a wide array of species.

Figure 11. Schematic representation of home ranges for two theoretical species projected on a road, and the distance between safe crossing opportunities (distance is equal to the diameter of their home range).

Figure 12. Schematic representation of home ranges for three individuals (x, y and z) with different locations for the center of each home range.