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A moose is trying to cross a busy road

Having the freedom to move toward resources and away from threats is critical to the health and wellbeing of all life on the planet, and it is foundational to the rights of self-determination for people and animals.

Through the social and technological infrastructures we create, humans restrict each other’s and other animals’ movements, both intentionally and unintentionally.

A 2018 study tracking 803 individuals in fifty-seven species of mammals found that the animals’ movements in areas of high human activity were reduced to one half to one third of the range they had in areas that had less human activity.

Although animal movement is restricted by a variety of human modifications of land, including through agriculture, fences, and dams, roads can be a major impediment to animal movement and can result in high rates of collision-related deaths.

The planning of human transportation infrastructure is far from being just, even if we only consider human interests. Historically, transportation planning has also severely neglected the rights, health, and wellbeing of animals.


The Rise of Road Ecology

It wasn’t until the 1990s that ecologists and other scientists began to seriously investigate the effects of roads and traffic on animals and ecosystems. The discipline of “road ecology” grew out of those investigations, soon giving rise to road ecology programs at a handful of universities around the world and to international conferences, created to conduct and share research to influence transportation policy.

Increasingly since the mid-1990s, researchers have collected data to determine how roadways and vehicular traffic have affected the movement of free-living animals, and the ways in which roadways and traffic alter the wellbeing and longevity of individual animals, the sizes of populations, the genetic diversity within populations, and the effects of animals on each other and on plant populations and soil health.

Road ecologists also study the effects of mitigation efforts.


The Effect of Roadways

Roads can affect the mobility of animals, both because many animals are either attracted to or repelled by road surfaces, noise, lights, chemical emissions, and other disturbances produced by traffic, and because of their ability or inability to avoid being hit by vehicles.

Studies have identified a few species that may either benefit from or not be affected by roadways. However, a 2009 analysis of seventy-nine empirical studies of the effects of roads on population size, population density, species presence, and/or species diversity found that the negative effects of roadways on animals outnumbered the positive effects by five to one, and outnumbered findings of no effect by two to one.

In the US, twenty-one threatened or endangered species have been identified as at increased risk of extinction because of roadways. They include eight species of mammals, seven of reptiles, three of amphibians, and three species of birds.


The Risk of Road Deaths

Roads can negatively affect individual animals and animal populations, either because of their vulnerability to environmental disturbances associated with traffic, as in the case of the disruption of songbird communication by traffic noise, or because of their vulnerability to road deaths.

Many species of animals are vulnerable to road deaths because they are attracted to or cannot avoid roads, and because they can’t sense or move quickly enough to avoid being hit by vehicles.

For example, some reptiles and amphibians are attracted to roads because of the warmth of road surfaces or because the texture of road materials resembles their natural habitats, and they consequently rest or lay their eggs on road surfaces.

Other reptiles and amphibians, as well as mammals and insects, are killed attempting to navigate roadways that fragment their habitats or cut across their migration routes.

Because they have large natural ranges and little aversion to roads or traffic, many large mammals are also particularly vulnerable to road deaths. These same species generally have low rates of reproduction, which means that road deaths can negatively impact their population sizes, which then affects other animals and plants in their ecosystems.

Humans are also frequently harmed in roadway collisions with animals, and transportation maintenance workers are put at risk when clearing carcasses from roadways.

In the US, a 2008 study by the Federal Highway Administration estimated that 26,000 injuries to humans and 200 human deaths a year are attributable to “wildlife-vehicle collisions” (WVCs) involving large animals. As large as those numbers are, they constitute just 4-10 percent of reported WVCs involving large animals, so the actual injury and death toll for all animals is even higher.

Such numeric studies of the impacts of roadways cannot document the mental and emotional distress caused to individual animals by the barriers to their movement; by air, noise, and light pollution; by the deaths of relatives and friends; and by their own injuries. These studies also don’t document the distress that humans experience from the losses they suffer and the compassion they feel for the animals that they or others injure or kill on roadways.


The Need for New Transportation Infrastructure Parameters

As with many other human activities, best practices for designing transportation infrastructure should include deeply considering and weighing the costs and benefits for humans and other animals equally, with the goal of

  • disrupting ecologies as little as possible to meet human needs;
  • employing only the most sustainable and regenerative technologies available; and
  • preserving and restoring as much habitat as is possible, despite the perceived financial costs.

The burden of proof to justify any new roadways should always be assigned to the would-be developing body and should show how the proposed roadways are necessary to benefit human and animal health and wellbeing.

Development policy should also operate based on the presumption that new development should be avoided whenever possible, and that no development will be permitted unless sufficient funds are allocated to cover the mitigation of negative effects to impacted animals and to vulnerable human individuals and communities.

Even in purely economic terms, analyses suggest that the benefits of animal-vehicle collision mitigation strategies outweighed the costs due to the collisions they prevent (see, for example, this discussion of the comparative cost analysis in Wyoming).

Roadway planners currently use a variety of measures to minimize the negative effects of roads on animals and to reduce the risk of human-animal collisions. These include

  • efforts to modify driver behavior, such as through
    • signage
    • animal detection sensors that alert drivers to the presence of large animals in the road
    • education campaigns encouraging compliance with lower speed limits
  • efforts to modify animal behavior, such as through
    • fencing
    • obstacles such as boulders placed along the edges of highways
  • location and design modifications, such as
    • avoiding particularly sensitive ecological areas
    • building long bridges or tunnels to minimize disturbances of animals
    • including crossings over or under roadways, which provides safe passage to animals

Of those planning strategies, animal crossings are the type most suited to preserving and restoring migration routes and mitigating other effects on animals of existing and future roadways.


The Benefits of Animal Crossings

Animal crossings include both tunnels under and bridges over roadways; they are sometimes called ecopassages or ecoducts to emphasize their importance in preventing or repairing ecosystem fragmentation.

Although not all attempts have been successful, many crossings have been shown to reduce animal fatalities by 80-90 percent. Best practices involve planning them with the preferences of the locally impacted species in mind—studies show that some animals prefer to use underpasses and others overpasses. And animal crossings appear to be most effective when used in combination with fencing that diverts animals away from roads and toward the passages.

Although animal crossings have been implemented sporadically around the world since the 1950s, their use has increased substantially starting in the mid-1990s.

Animal crossings that are now in effect across roadways include those designed to provide passage for

  • salamanders, turtles, tortoises, alligators, panthers, bears, mountain lions, elks, moose, pronghorns, mule deer, mountain goats, and pikas in the US
  • moose, elks, and grizzly bears in Canada
  • bandicoots, wallabies, and crabs in Australia
  • jaguars in Mexico
  • gold monkeys and pumas in Brazil
  • elephants in Kenya
  • badgers, deer, and wild boars in the Netherlands
  • badgers and voles in the UK
  • toads in Wales
  • pangolins, civets, and macaques in Singapore

Similar crossings have been built in some locations across rail lines.

Major animal crossings are currently being planned in Sweden to assist reindeer, moose, and lynxes, and in southern California to prevent the regional extinction of mountain lions.

In the US, recognition of the importance of animal crossings has grown as their effectiveness has been proven in several states.

In 2019 and 2020, bills introduced separately in the US Senate and in the House of Representatives would have allocated funding for projects such as developing a national animal crossing plan on federal and tribal lands and reducing animal-vehicle collisions along national highways.

In the absence of federal funds, animal crossing construction in the US has depended on state transportation budgets and on donations from conservation nonprofits and private individuals.


an example of an animal crossing, seen from above
An example of an animal crossing, seen from above.


The Expansion of Animal Crossings in the US

In the US, the state of Wyoming was one of the earliest to commit to extensive animal crossings to prevent animal-vehicle collisions and to restore the migration routes of animals.

After initial failed attempts to reduce collisions between vehicles and mule deer by using roadside fencing and flashing lights to alert drivers, an underpass was constructed on Highway 30 through Nugget Canyon in 2001. Its success in allowing the safe passage of mule deer, antelopes, elks, and other animals led to the construction of six additional underpasses on a twelve-mile stretch of the highway, which led to an 80 percent drop in reported animal deaths.

Webcam footage from October through December 2010 documented more than 6000 mule deer moving south through the underpasses and approximately 400 moving north. A webcam in an underpass completed in September 2009 along Highway 789 near Baggs showed more than 3000 deer using it between September and January.

In 2012, eight underpasses and overpasses, with directive fencing, were installed across US Highway 191. These crossings facilitate the movement of a herd of pronghorns along their 150-mile, millennia-old seasonal migration route, aiding in the extension of the first federally-designated migration corridor, known as the Path of the Pronghorn.

A first-of-its kind federal grant from the US Department of Transportation, awarded in November 2019, will provide partial support for the construction of underpasses along a nineteen-mile stretch of US 189 to facilitate the movement of mule deer, pronghorns, and moose.

One of the newest crossings in the US is a land bridge in San Antonio that opened in December 2020. An ecopassage that includes native plantings, it connects segments of a 330-acre park that were previously divided by a highway. Designed in compliance with Americans with Disabilities Act standards, it is intended to be used by human visitors to the park, as well as by white-tailed deer, coyotes, ring-tailed cats, armadillos, and other animals who live in or traverse the park.

A similar project currently underway in Houston will create a land bridge for animal and human use that connects two parts of a 1500-acre park.

Unfortunately, even in such planning, the rights of individual animals are subordinated to human economic interests. While the wellbeing of animals is often emphasized as part of the rationale for such projects, too often part of the justification is a concern about the risk of lost revenue from hunting and tourism.

Similarly, the reindeer who will use the planned overpass in Sweden are members of herds under the control of humans, and at least one overpass in Wyoming, constructed to facilitate the migration of pronghorns, is also used by ranchers to herd cows.


The Need to Include Animal Wellbeing in Transportation Planning

In addition to creating animal crossings to mitigate the negative effects of existing roadways and incorporating ecopassages in all future roadway plans, entities involved in planning must commit funds to study the effectiveness of various designs to ensure that they best meet the goals of enhancing animal and human wellbeing.

For example, recent research in the Netherlands found that overpasses for large animals, although not as economical, created more ecological gains than did underpasses or overpasses designed for both human and animal use.

A recent Spanish study on animal crossings concluded that wider crossings that include a variety of ecological structures may be required to prevent passages from becoming traps for animals who are already vulnerable to predation. And a recent study in France of railway underpasses found that acoustic enrichment can enhance their use by amphibians.

As infrastructure development continues in communities across the globe, it is critical that all decisions consider the rights, health, and wellbeing of vulnerable people and animals.



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