Best Practices Manual: Wildlife Vehicle Collision Reduction Study
CHAPTER 5: DESIGN AND GUIDANCE OF MITIGATIONS FOR THREATENED AND ENDANGERED SPECIES
This chapter covers mitigation methods focused on reducing WVCs involving threatened and endangered species (figure 101). In the Report to Congress, 21 Federally listed threatened and endangered species were identified in the United States for which direct road mortality is among the major threats to the survival of the species or certain populations of that species (table 8).
Figure 101. Mitigations for threatened and endangered species as part of a strategy for reducing WVCs.
In this chapter existing mitigation measures for the 21 species and species groups listed in table 8 are discussed. In addition to existing measures, additional measures are suggested for each species or species group that may help reduce collisions with vehicles. It is important to note that while direct road mortality is among the major threats to these 21 species, it is not necessarily the only or even the most important threat to these species. The survival of the species listed in this chapter is also threatened by other factors, including:
- Habitat loss due to agriculture, urbanization, mining, and changes in hydrology.
- Reduced habitat quality due to agricultural and silvicultural practices such as livestock grazing, logging, fire suppression, introduction of non-native plant species, and water contamination by pesticides and other pollutants.
- Habitat fragmentation due to roads or other unsuitable habitat.
- Competition and predation by non-native species.
- Other sources of natural and unnatural mortality such as off-road vehicles, poaching, direct killing or collection by humans, or disease.
- Low recruitment and loss of genetic diversity due to small population.
This implies that a substantial reduction in road mortality is not necessarily sufficient for the recovery of the species listed in this chapter. For successful species recovery, including mitigation for effects related to roads and traffic, it is advisable to use an integrated approach (e. g. , see Brown43). For more information about the road mortality numbers of the species listed in table 8, please consult the Report to Congress.
||California tiger salamander (Ambystoma californiense), C. CA, S. Barb. , Son. county
||Audubon's crested caracara (Polyborus plancus audubonii), FL pop.
||Flatwoods salamander (Ambystoma cingulatum)
||Hawaiian goose (Branta sandvicensis)
||Houston toad (Bufo houstonensis)
||Florida scrub jay (Aphelocoma coerulescens)
||American crocodile (Crocodylus acutus)
||Lower Keys marsh rabbit, (Sylvilagus palustris hefneri)
||Desert tortoise (Gopherus agassizii), except in Sonoran Desert
||Key deer (Odocoileus virginianus clavium)
||Gopher tortoise (Gopherus polyphemus), W of Mobile/Tombigbee Rs.
||Bighorn sheep, Peninsular CA pop.(Ovis canadensis)
||Alabama red-bellied turtle (Pseudemys alabamensis)
||San Joaquin kit fox (Vulpes macrotis mutica)
||Bog turtle (Muhlenberg) northern population (Clemmys muhlenbergii)
||Canada lynx (Lynx canadensis), lower 48 States
||Copperbelly water snake (Nerodia erythrogaster neglecta)
||Ocelot (Leopardus pardalis)
||Eastern indigo snake, eastern indigo (Drymarchon corais couperi)
||Florida panther (Felis concolor coryi)
||Red wolf (Canis rufus), except where XN
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5.1. AMPHIBIANS AND REPTILES
Relatively few mitigation measures were identified that were specifically targeted at one or more of the 10 species of amphibians and reptiles listed in table 8.Mitigations specifically aimed at the American crocodile, desert tortoise, gopher tortoise, and Alabama red-bellied turtle are summarized in this section.
5.1.1. American Crocodile
There are several mitigations in Florida that target the American crocodile in southern Florida. Warning signs for the crocodiles were erected during road construction projects on U.S. Highway 1 between Florida City and Key Largo in January, 2008 (figure 102).
Figure 102. A warning sign for American crocodiles along U.S. Highway 1 between Florida City and Key Largo, FL (copyright: Marcel Huijser).
The widening of the road was done in combination with the installation of wildlife fencing, which specifically targeted the American crocodile and was intended to discourage them from entering the highway corridor (figure 103).
Figure 103. A 1.8.3-m-high (6-ft-high) chain-link wildlife fence intended for American crocodile along U.S. Highway 1 between Florida City and Key Largo, FL (copyright: Marcel Huijser).
In addition to the fencing, between 12 and 20 underpasses (about 2.5 m (8.2 ft) wide) were installed specifically for the crocodiles to use (personal communication, Steve Klett, Crocodile Refuge manager, Key Largo, FL). In the underpass shown in figure 104, the 1.8.3 m (6 ft) fence can be seen in the upper left corner where it ties in with the underpass.
Figure 104. An underpass for American crocodiles along U.S. Highway 1 between Florida City and Key Largo, FL (copyright: Marcel Huijser).
On Highway 905A just south of the Card Sound Bridge in Key Largo, eight to ten culverts have been installed for the American crocodile (personal communication, Steve Klett, Crocodile Refuge manager, Key Largo, FL). These culverts are about 1.8.m (6 ft) wide, and there are short sections of fencing 30 m (100 ft) long and about 90 cm (3 ft) high to guide American crocodiles to these structures (figure 105). However, the crocodiles appear to be using these underpasses less than it was hoped because vegetation (mainly mangroves) obstructs the entrances. There are no data on effectiveness of these signs, fences or culverts.
Figure 105. A short section of fencing (30 m (100 ft) long, 90 cm (3 ft) high) aimed at directing American crocodiles to 1.8.-m-wide (6-ft-wide) culverts along Highway 905A, just south of the Card Sound Bridge in Key Largo (copyright: Marcel Huijser).
The fences shown in figure 105 used to be bent at a 45-degree angle away from the road, but right-of-way mowing equipment bent them almost straight again. Note the mangrove vegetation on the right that appears to obstruct the entrance to the culverts.
5.1.2. Desert Tortoise
Mitigation measures for desert tortoises have been installed in California. They include desert tortoise warning signs, such as the one depicted in figure 106, and desert tortoise fencing in combination with retrofitted culverts.
Figure 106. Desert tortoise warning sign along an access road (no name) to the Hyundai Proving Grounds, near Mojave, Kern County, CA (copyright: William I. Boarman, CSRC).
Fences (figure 107) designed to prevent tortoises from crossing the road resulted in 93 percent fewer tortoise roadkills. 441
The fence extends to 46 cm (18 in) above the ground, and is buried 15 cm (6 in) below ground. 45 The fence consists of 0.64 cm (1/4 in) mesh hardware cloth, which was a barrier to all but the smallest species in the study area. 45
More recently, 2.5 x 5 cm (1 x 2 in) hog wire (with the 2.5 cm being the horizontal dimension and 5 cm the vertical) has been used more often (but it remains unstudied) because it is less expensive and is not a substantial barrier to smaller non-target species (personal communication, William Boarman, Conservation Science Research and Consulting, Spring Valley, CA).
However, those smaller animal species still access the road corridor where they are susceptible to road mortality. The mesh fence was attached to a five-strand barbed-wire fence, with the bottom two strands above the mesh wire fence left unbarbed to allow easy crossing by coyotes, kit foxes, and rabbits. 45
Figure 107. barrier fence erected along State Highway 58 near Kramer Junction, San Bernardino County, CA (copyright: William I. Boarman, U.S. Geological Survey).
The fence tied into or went above existing culverts, which were located for drainage and not for tortoise movements (figure 108). These culverts (24 total) vary in size, but they are typically made from corrugated metal and are 142-163 cm (56-64 in) in diameter and 48-63 m (158-208 ft) long (personal communication William Boarman, Conservation Science Research and Consulting, Spring Valley, CA). The entrances of the culverts were flush with the ground, and small boulders were installed to prevent undercutting the apron. These small boulders were covered with soil so that desert tortoises could access the culverts.
Figure 108. Desert tortoise exiting a culvert retrofitted to allow tortoises to cross beneath State Highway 58 near Kramer Junction, San Bernardino County, CA (copyright: William I. Boarman, U.S. Geological Survey).
5.1.3. Gopher Tortoise
Fences and culverts were installed along a section of Highway 63 in Green County, south of Leakesville, Mississippi, to reduce gopher tortoise road mortality (personal communication, Matthew J. Aresco, Nokuse Plantation, Bruce, FL; Claiborne Barnwell and Chuck Walters, Mississippi Department of Transportation).
The aim of the mitigation measures is to allow gopher tortoises to cross under the road (figure 109). Highway 63 has 24.1 km (15 miles) of gopher tortoise fencing and, because of the nature of the terrain, there is only one culvert (between Lucedale and Leakesville) that was specifically designed for the gopher tortoise. At this culvert the fence stretches out about 914 m (3,000 ft) on each side. Some of the fencing was installed as early as 1998, and along those road sections the number of reported road-killed gopher tortoises was reduced from one to two per year to zero (personal communication, Chuck Walters, Mississippi Department of Transportation).
Figure 109. Fences lead gopher tortoises toward a culvert along Highway 63 in Green County, south of Leakesville, MS (copyright: Chuck Walters).
5.1.4. Alabama Red-Bellied Turtle
Mitigation measures to reduce road mortality for the Alabama red-bellied turtle were installed along the Mobile Bay Causeway (U.S. Highway 90/98) between Spanish Fort and Mobile. Phase 1 of the project was still ongoing in April, 2008, and involved 4.2 km (2.6 mi) of chain-link fence (personal communication, David Nelson, Department of Biological Sciences, University of South Alabama) (figure 110). Most of the fence is on the north side of the road where there is fresh water rather than the brackish or salt water on the south side of the road.
Figure 110. Fences keep Alabama red-bellied turtles from entering the roadway and lead them toward bridges where they can cross under the road (Source: Al DOT)
5.1.5. General Amphibian and Reptile Mitigations
- Mitigation measures used for other amphibian and reptile species that are not necessarily endangered or threatened include screens, 40-50 cm (16-20 in) above ground and about 10 cm (4 in) underground, placed alongside the road that guide amphibians toward underpasses (figure 111-119).
- Smooth plastic screens are preferred over mesh wire as some amphibians can climb the mesh wire and small individuals may go through the mesh. Mesh wire fences also are often damaged during vegetation maintenance practices. Screens that are curved back to the side away from the road are not recommended because of problems with vegetation and right-of-way maintenance.
- It is critical that the fence screens fit tightly to the underpasses.
- The fences are most successful if they do not deflect amphibian movements by more than 60 degrees. Thus the orientation of the fences in relation to the direction of movement of the amphibians is important (figure 114).
Figure 111. Concrete barrier leads amphibians toward an underpass along the road "Hilversumsestraatweg" near the town Hilversum, The Netherlands (copyright: Marcel Huijser).
Figure 112. Plastic sheets that guide amphibians to an underpass, near the town Hilversum, The Netherlands (copyright: Marcel Huijser).
Figure 113. Plastic screens combined with a badger fence along the road "Hilversumsestraatweg" near the town Hilversum, The Netherlands (copyright: Marcel Huijser).
Figure 114. Plastic screens combined with a badger fence guiding the animals towards an underpass for amphibians under the road "Hilversumsestraatweg" near the town Hilversum, The Netherlands (copyright: Marcel Huijser).
Figure 115. Concrete barrier and planks guiding amphibians toward an underpass under the road "Bussumergrindweg", near the town Hilversum, The Netherlands (copyright: Marcel Huijser).
Figure 116. Plastic screens combined with a medium- and large-mammal fence guiding the animals toward an overpass (Waterloo) along the A73 motorway near the town Roermond, The Netherlands. Note that the screw that holds the two plastic sheets together is broken, allowing for a gap and potential intrusions of amphibians and other small species into the road corridor (copyright: Marcel Huijser).
Figure 117. Plastic screens combined with a medium- and large-mammal fence guiding the animals toward an overpass (Waterloo) along the A73 motorway near the town Roermond, The Netherlands. Note that two adjacent sheets have a gap that is covered by fine mesh wire fence to prevent amphibians and other small species from entering the road corridor (copyright: Marcel Huijser).
Figure 118. Plastic screens that collapsed were intended to guide amphibians toward an underpass near the town Hilversum, The Netherlands. The gap allows amphibians and other small species to enter the road corridor (copyright: Marcel Huijser).
Figure 119. The vegetation on both sides of a concrete barrier for amphibians was mowed to prevent small species from using the vegetation to climb the barrier and enter the road corridor, near the town Haywards Heath, West Sussex, England (copyright: Marcel Huijser).
- It is important that soil and air humidity inside the underpasses are similar to that of the surroundings. Light conditions may be important as well. Therefore, open top structures are usually recommended (figure 120, 121, 122 and 123).
- Rectangular structures are preferred over round or half-round structures because of the increased openness of the top of the structure and associated physical conditions inside the structure. In addition, rectangular structures are harder to climb for amphibians than round or half-round structures, and fences can connect more tightly to rectangular structures.
- Design to prevent the accumulation of standing water inside of the underpasses.
Figure 120. Open tops for underpasses for amphibians are often recommended so that air, soil humidity and light conditions inside the tunnels are similar to conditions outside (reprinted with permission from Kruidering et al. , 2005). 27
Figure 121. The open concrete top of an underpass for amphibians, allowing light, air, and moisture to enter, near the town Hilversum, The Netherlands (copyright: Marcel Huijser).
Figure 122. The open metal top of an underpass for amphibians, allowing light, air, and moisture to enter, under the road "Hilversumsestraatweg" near the town Hilversum, The Netherlands (copyright: Marcel Huijser).
Figure 123. Close-up of an underpass for amphibians along the road "Hilversumsestraatweg" near the town Hilversum, The Netherlands. Note that the open roof allows light, air, and moisture to enter the underpass (copyright: Marcel Huijser).
- For suggested minimum dimensions of underpasses for amphibians see table 9.
Table 9.Recommended minimum dimensions (width x height or diameter) for amphibian crossing structures (adapted from Kruidering et al. , 2005). 27
||Length structure (road width)
|< 20 m
|Rectangular (box) culvert
||1.0 x 0.75 m
(3.3 x 2.5 ft)
|1.5 x 1.0 m
(4.9 x 3.3 ft)
|1.75 x 1.25 m
(5.7 x 4.1 ft)
|2.0 x 1.5 m
(6.6 x 4.9 ft)
|2.25 x 1.75 m
(7.4 x 5.7 ft)
||1.0 x 0.7 m
(3.3 x 2.3 ft)
|1.4 x 0.7 m
(4.6 x 2.3 ft)
|1.6 x 1.1 m
5.2 x 3.6 ft)
- Similar to fencing for large animals (section 4.2), mitigation measures for reptiles may consist of concrete barriers (figures 124 and 125). These types of barriers are typically very effective for most amphibian and reptile species except tree frogs, which were able to climb the barrier. Because of the climbing ability of many amphibians and reptiles, an overhang is recommended (figure 126).
Figure 124. A concrete barrier for amphibians and reptiles along U.S. Highway 441 through Paynes Prairie, south of Gainesville, FL (copyright: Marcel Huijser).
Figure 125. A concrete barrier combined with an underpass for amphibians and reptiles along U.S. Highway 441 through Paynes Prairie, south of Gainesville, FL (copyright: Marcel Huijser).
Figure 126. A closeup of the concrete barrier for amphibians and reptiles along U.S. Highway 441 through Paynes Prairie, south of Gainesville, FL (copyright: Marcel Huijser).
- Breaks in the barrier at access roads may be mitigated by gates (figures 127 and 128). To prevent amphibians and small reptiles from crawling under the fence, the fence must connect closely to the ground (figure 127). Alternatively, flaps that hang down from the bottom of the gate can be installed (figure 128).
Figure 127. A gate that allows for access to a field with crops through an amphibian barrier, and a medium- and large-mammal fence, near wildlife overpass "Schinheuvel," near the town Roermond, The Netherlands. Note that the gate connects closely to a hard surface to prevent amphibians and other small species from entering the road corridor. (copyright: Marcel Huijser).
Figure 128. A gate for pedestrians through a badger fence and an amphibian barrier. Note that the gate has sheeting at the bottom to prevent amphibians and other small species from entering the road corridor. Note also that the gate is angled, so that gravity will automatically close the gate (copyright: Marcel Huijser).
- Suitable habitat for amphibians may also be created on overpasses designed for multiple species. The habitat in figure 129 has a pump installed that pumps water to the center of the overpass, creating a wet zone that connects with ponds on both sides. The great crested newt, an endangered species in The Netherlands, was one of the target species for this connectivity project. Great crested newts were found on top of the overpass within a few months of completion. The square wooden boards shown in figure 129 are for monitoring amphibians on the overpass. During the day the amphibians crawl under the wooden boards for cover, and researcher lift the boards regularly to check for their presence.
Figure 129. The wildlife overpass "Groene Woud" in The Netherlands, with amphibian habitat (copyright: Marcel Huijser).
- In some cases, instead of guiding amphibians toward crossing structures, screens are installed to lead the animals to pitfalls. The animals are trapped in these pitfalls until volunteers collect them and carry them to the other side of the road. These activities, including the presence of the screen fence, are usually restricted during spring migration.
- Temporary road closures, such as during warm, wet, spring nights, are sometimes used to reduce amphibian road mortality. These road closures usually involve small low-volume roads.
- Guardrails can be painted black to reduce glare and light disturbance for amphibians (figure 130).
Figure 130. Guardrail painted black near amphibian tunnels and a multiple-use underpass, along the road "Hilversumsestraatweg", near the town Hilversum, The Netherlands. The painted guardrail is expected to reduce glare and light disturbance for amphibians and other species groups (copyright: Marcel Huijser).
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5.2.1. Hawaiian Goose (Nene)
- Mitigation measures for the Hawaiian goose, or Nene, consist of roadside warning signs, advisory speed limit reductions, "do not feed" signs (especially alongside roads or in parking areas), and speed bumps (figure 131 through 134). There are no data available on the effectiveness of these signs.
Figure 131. Hawaiian goose warning sign in Volcanoes National Park, HI (copyright: Marcel Huijser).
Figure 132. Hawaiian goose warning sign in combination with advisory speed limit in Volcanoes National Park, HI (copyright: Marcel Huijser).
Figure 133. Hawaiian goose "no feeding" sign in Kokee State Park, Kauai, HI (copyright: Marcel Huijser).
Figure 134. Speed bump, specifically installed to reduce vehicle speed for nearby breeding, roosting and feeding area for the Hawaiian goose in Volcanoes National Park, HI (copyright: Marcel Huijser).
- Other mitigation measures that may be considered for the Hawaiian goose include at-grade and below-grade (underpasses) safe crossing opportunities. The geese do not fly when they change feathers or when they protect and guide the goslings between breeding, roosting, and feeding sites. This makes them vulnerable to road mortality.
- Roads that are lower than surrounding areas, and berms, poles, shrubs or trees adjacent to roads have been suggested to encourage birds to fly higher than passing vehicles (figures 135, 136, and 137). In 1994, a bridge along state route A-1-A over the Sebastian inlet in Florida was equipped with 122 poles, each 3m (9.8.ft) long and 3.7m (12.1 ft) apart (figure 135). These bridge poles reduced vehicle collisions with royal terns and brown pelicans by 64 percent. 46
- The trees that encourage birds to "hop over" the road above traffic should be sufficiently tall, and shrubs close to the road should be removed (figure 136). Furthermore, lower branches should be trimmed at the trunk to discourage birds from spending time at lower heights close to the road.
- For raptors and other scavengers, the removal of road-killed animals reduces the risk of collisions with vehicles.
Figure 135. Poles across State Road A-1-A over the Sebastian Inlet in Florida (copyright: Marcel Huijser).
Figure 136. "Hop-over" for birds to encourage them to fly above traffic (reprinted with permission from Kruidering et al. , 2005). 27
Figure 137. Embankment for birds to encourage them to fly above the traffic on the A27 motorway near Huizen, The Netherlands. Embankments are on both sides of the road, and also serve to reduce sound and light disturbance into the open grasslands. Note that the yellow top of a freight truck shows just above the embankment (copyright: Marcel Huijser).
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Lower speed limits, general information signs, signs warning of animals on the road, rumble strips to increase driver alertness, wildlife fencing and wildlife under- and overpasses have all been applied to one or more mammal species listed in table 8.
In general, wildlife fencing combined with under- and overpasses are considered the most effective and robust mitigation measure for medium- and large-sized mammals. Animal detection systems are also an option but they should still be considered experimental.
5.3.1. Florida Key Deer
Species specific examples of unique signing have been implemented for Key deer in Florida (figures 138 to 141). In addition to warning signs, a lower nighttime speed limit is posted on U.S. Highway 1 on Big Pine Key.
Figure 138. Warning signs for Key deer along the two-lane U.S. Highway 1 on Big Pine Key, FL (copyright: Marcel Huijser).
Figure 139. Signs showing the road mortality numbers of Key deer on a side road of U.S. Highway 1 on Big Pine Key (copyright: Marcel Huijser).
Figure 140. Warning sign for Key deer combined with message to not feed the deer along a side road of U.S. Highway 1 on Big Pine Key (copyright: Marcel Huijser).
Figure 141. Permanently flashing warning signs for Key deer along U.S. Highway 1 on Big Pine Key (copyright: Marcel Huijser).
Fencing similar to that discussed in section 4.2 has been used for Key deer in Florida (figure 142). The fencing is shorter than that used for other deer because of the abilities of the target species.
Figure 142. A 1.8.-m-high (6-ft-high) fence for Key deer along U.S. Highway 1 on Big Pine Key (copyright: Marcel Huijser).
At locations where the road surface was at least as high as the fence (1.8.m (6 ft)), the fence was replaced by the concrete wall of the roadbed (figure 143).
Figure 143. The approach to an underpass for Key deer along U.S. Highway 1 on Big Pine Key (copyright: Marcel Huijser).
An example of an underpass used for Key deer is shown in figure 144.For more detail on underpasses and overpasses refer to sections 4.3 and 4.4.
Figure 144. One of the two underpasses for Key deer along U.S. Highway 1 on Big Pine Key (copyright: Marcel Huijser).
5.3.2. Florida Panther
Reduced speeds, warning signage, fencing and underpasses have been used in mitigation efforts related to the Florida panther. Figures 145 through 150 show some of the unique warning signs used.
Figure 145. Warning sign for the Florida panther along SR 29 in southern Florida (copyright: Marcel Huijser).
Figure 146. Warning sign for the Florida panther along SR 29 in southern Florida (copyright: Marcel Huijser).
Figure 147. Warning sign for the Florida panther along SR 29 in southern Florida (copyright: Marcel Huijser).
Figure 148. Warning sign for Florida panther in southern Florida (copyright: Marcel Huijser).
Figure 149. Warning signs for Florida panther along SR 29 in southern Florida (copyright: Marcel Huijser).
Figure 150. Permanently flashing warning sign for Florida panther, combined with a rumble strip along SR 29 in southern Florida (copyright: Marcel Huijser).
- Fencing (figure 151) and crossing structures (figure 152) have been used for the Florida panther. For more detail on fencing refer to section 4.2.For more detail on crossing structures refer to sections 4.3 and 4.4.
Figure 151. Wildlife fencing, with barbed wire overhang, for the Florida panther on SR 29 in southern Florida (copyright: Marcel Huijser).
Figure 152. Underpass for hydrology (water flow) and the Florida panther along Interstate 75 in southern Florida (copyright: Marcel Huijser).
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Questions and feedback should be directed to Marlys Osterhues (firstname.lastname@example.org, 202-366-2052).