Wildlife Crossings: Bridging the Gap between Wildlife and Public

Written by Angela Theresa Tiongson
Illustration by Sabrina T. Laceda
Published 2021 November 29

Road and Highway construction negatively impacts wildlife through habitat loss and fragmentation, the introduction of an additive source of mortality, and the disruption of animal movement.[1] The continuous expansion of road networks that traverse across terrestrial ecosystems cause major habitat degradation and fragmentation.[2] Paved roads act as a restriction that separates wildlife populations across habitat fragments.[2] Furthermore, roaring traffic does not discourage big mammals such as moose and bears from crossing roads—nor does it prevent smaller animals from being run over by speeding vehicles.[3] Wildlife mortalities caused by wildlife-vehicle collisions have been the most widely known impact of roads over the last three decades.[4] Wildlife road accidents can also affect public safety, especially when large mammals are involved.[4] In contrast, amphibians in general, seem to be more vulnerable to wildlife-vehicle collisions relative to other taxa.[4] These animals attempt to traverse roads during their spring migration switching from terrestrial hibernacula to aquatic breeding habitats.[4] Freshwater turtles are considered as one of the most affected species by vehicle collisions due to the proximity of roads to bodies of freshwaters.[2] Their cryptic coloration also makes them unnoticeable and thus more vulnerable during the nighttime.[2] Moreover, among reptiles, researchers have stated that female turtles leave the water to lay their eggs and are crushed by vehicles in the process.[5] The increasing incidents of wildlife road mortalities have prompted scientists and conservationists to come up with a solution to mitigate these fatalities. One solution, however, has been found to be remarkably effective in decreasing wildlife road fatalities: wildlife crossings.[3] 


Wildlife crossing combined with fencing that guides animals under or over highways reduce around 85 to 95 percent of collisions.[3] It was first conceptualized and developed in France during the 1950s and was later adapted in the Netherlands where more than 600 crossings have since been constructed.[6] The longest known animal crossing, the Natuurbrug Zanderij Crailoo, was built by the Dutch.[6] It spans more than 0.8 kilometers (0.5 miles).[6] Wildlife crossings have also been implemented in Australia, Canada, and other parts of the world.[6] Among the earliest U.S. crossings was built in Davis, California in 1995.[6] It was a six-inch-wide tunnel or “ecoduct” that allowed frogs to pass through and reach the wetland on the other side of the road.[6] From 1996 up to 2016, six bridges and 38 underpasses were built for wildlife to traverse the Trans-Canada Highway, which divides the Banff National Park in Alberta, Canada.[6] More than 150,000 recorded crossings were made by elk, moose, black bears, cougars, and grizzly bears.[6] The Park reported an 80 percent decrease in motor accidents involving wildlife.[6] The koala population in Queensland, Australia has been decreasing at a frightening rate over the past few decades.[6] A government report states that among the major culprits of koala deaths were Auto accidents in southeast Queensland from 997 up to 2011.[6] This prompted their state government to implement around half a dozen wildlife crossings between 2010 and 2013.[6] Most of the work done was modifications of existing drainage tunnels under highways.[6] An ecologist observing the sites was surprised at the quick adaptation of koalas to the utilization of these tunnels.[6]


Under-crossings that span beneath highways assist shyer and smaller animals that may have gone unnoticed to drivers on the road.[3] Species such as gold monkeys and pumas in Brazil and even water voles in London prefer to travel through these under-crossings.[3] Washington State has recently started constructing its first wildlife bridge in 2015 located east of the Snoqualmie Pass in the Cascades along Interstate 90.[3] Although it’s only a set of arches presently, deer and coyotes have started using the overpass.[3] Including the six underpasses built along I-90, these passes will allow elk, black bears, mountain lions, pika, and even trout to cross what was once an impenetrable barrier of the road. [3]

Wildlife crossings come in a variety of forms which depend on the target species and the geographic features of the land.[6] The most common structures are bridges and overpasses, tunnels, viaducts, and culverts—all of which serve the same purpose, which is to reduce collisions, provide a safe pathway for animals and reconnect wildlife habitats.[6] Wildlife bridges are usually coated with native vegetation.[6] In the United Kingdom, they are referred to as “green bridges.”[6] The intention is to make the crossing appear natural and less intimidating to encourage usage among animals.[6] An effective wildlife passage structure will need assessment on the features that will optimize their utilization. This includes careful consideration of the different aspects that would comprise a wildlife crossing: placement, size, light, moisture, temperature, noise, substrate, approaches, fencing, human disturbance, and interactions among species.[7] Although it would be unfeasible to design a specific passage structure to accommodate specific species, there are existing strategies that are practical solutions that are inclusive to a larger variety of animals.[7] One of such is the wildlife overpasses which are currently present in the Europe, U.S., and Canada.[7] 50m to 200m wide overpasses are said to be more efficacious and accommodating in general relative to underpasses.[7] In comparison, they are less confining, quieter, maintain ambient conditions of rainfall, temperature and light, and can also serve as an intermediate habitat for small creatures such as reptiles, amphibians, and small mammals.[7] The only disadvantage is costly and less accommodating for semi-aquatic species such as muskrats, beavers, and alligators.[7] Another good consideration would be surveying roadkill hotspots as an important baseline for site-specific implementation of methods wildlife road mortality mitigations.[8] 


Although a lot of places around the world have adopted this method of wildlife road mortality mitigation, the Philippines is yet to catch on. Preceding investigations of wildlife road mortalities along the national highway in Central Palawan reported an alarming population of road-killed bird and mammal species.[9] Furthermore, a recent study conducted along the national highway segment between Puerto Princesa City and the municipality of Aborlan, Palawan observed 127 road-killed incidents of freshwater turtles.[2] The study also pointed out that roadkill hotspots along a certain road suggested its proximity to bodies of water. This observation supports that of Langen who claims that the presence of bodies of water along highways are critical predictors of road mortality among three species of freshwater turtles.[8] Not only turtles but wild monkeys and other animals are frequently seen on the roadsides of mountainous regions in the Philippines. Thus, with the continuous expansion of road networks, the community also must keep in mind any form of wildlife road mortality mitigations such as wildlife overpasses, bridges, culverts, and etc. 



  1. Andrews, A. 1990. Fragmentation of habitat by roads and utility corridors: a review. Aust. Zool. 26(3&4):130-141.
  2. Bernardo Jr AA. 2019. Road mortality of freshwater turtles in Palawan, Philippines.
  3. Vartan, S. (2019, April 16). How Wildlife Bridges over Highways make Animals—and people—safer. National Geographic. Retrieved from https://www.nationalgeographic.com/animals/article/wildlife-overpasses-underpasses-make-animals-people-safer
  4. Bennett, V.J. Effects of Road Density and Pattern on the Conservation of Species and Biodiversity. Curr Landscape Ecol Rep 2, 1–11 (2017). https://doi.org/10.1007/s40823-017-0020-6
  5. Tok CV, Ayaz D, Cicek K. Road mortality of amphibians and reptiles in the Anatolian part of Turkey. Turk J Zool. 2011;35(6):851–7. doi:10.3906/zoo-0911-97.
  6. National Geographic Society. 2019. Wildlife Crossings. Retrieved from https://www.nationalgeographic.org/article/wildlife-crossings/7th-grade/
  7. Jackson, S.D. and C.R. Griffin. 2000. A Strategy for Mitigating Highway Impacts on Wildlife. Pp. 143- 159 In Messmer, T.A. and B. West, (eds) Wildlife and Highways: Seeking Solutions to an Ecological and Socio-economic Dilemma. The Wildlife Society.
  8. Langen TA, Gunson KE, Scheiner CA and Boulerice JT. 2012. Road mortality in freshwater turtles: identifying causes of spatial patterns to optimize road planning and mitigation. Biodiversity and Conservation, 21(12): 3017-3034. 
  9. Bernardo A Jr. 2011. Vehicle-induced mortalities of birds and mammals between Aborlan and Puerto Princesa City National Highway. The Palawan Scientist, 5(1): 1-10.

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