The Impact of Highways on Wildlife

Lauren Richie

The United States is divided by at least 3.9 million miles of public roads, about three quarters of which are in rural areas (5). This translates into approximately 20 million acres of lost wildlife habitat, an area comparable to that of South Carolina (4, 5). The actual area is likely much greater, as these numbers do not include private roads, driveways, nor parking lots (5). More than 200 million vehicles travel almost 8 billion miles every day in the U.S., causing an estimated 725,000-1,500,000 collisions between vehicles and ungulates (large hoofed animals) every year (1). The impact of roads on the ecology of natural habitat is estimated to affect 15-20% of the United States’ land (5).

Highways affect wildlife most obviously and visibly to humans through road kill. Millions of animals are struck and killed by a traveling car every year in the U.S., including mammals, birds, reptiles, and amphibians (4). The number may be much greater, as our estimates of the magnitude of road kill are limited by poor record keeping (5). Certain species are threatened by highway traffic more than others (4). For example, species such as turtles and salamanders are too slow-moving to move out of the way of cars, and animals that travel large distances such as wolves, grizzly bears, and mountain lions have to cross roads daily. The population of the endangered ocelot has been reduced to a mere 80 individuals, in part due to road kill.

Roads also greatly reduce the amount of habitat available for wildlife, and degrade the quality of nearby remaining habitat (4, 5). The land adjacent to roads is disrupted by noise, air and light pollution, and disturbance, which worsens the problem of invasions by exotic plant species (5). Construction of roads is particularly destructive to wildlife (5). It causes significant erosion on adjacent land, and storm water runoff carry soil, organic matter, and chemicals to aquatic systems, often with detrimental effects. Approximately 10 million tons of rock salt were dumped on roads in the U.S. between the mid-1980s and mid-1990s, according to a 1996 report by the EPA (5). This salt is also carried to aquatic systems, and can seriously pollute freshwater and affect many organisms by traveling through the food chain.

Habitat loss has a particularly severe effect on threatened and endangered species (4). For example, the grizzly bear, classified as threatened, is restricted to an area that is less than two percent of its original range south of Canada. The scarcely seen lynx, once found across the northern U.S., is now limited to only two healthy populations. Amphibians such as the red-legged frog and the Arroyo toad are found in fractions of their original ranges in California. Attwater’s greater prairie chicken is struggling at a total population of 42 in Texas.

Another major way in which highways threaten wildlife is by fragmenting habitat (4). This separates populations into isolated groups and hinders movement and reproduction between them, which are important for replenishing small populations that are vulnerable to natural disturbances and predation (5). Isolated populations are also more susceptible to inbreeding and the consequent loss of genetic diversity (4). A large number of studies examining the effects of fragmentation on wildlife and ecosystem ecology have found that wildlife corridors and connectivity are beneficial for wildlife movement and biodiversity (5). Highways can be enough of an obstruction that animals are prevented from reaching locations important to part of their life cycle or food requirements, such as a water source (5).

The fatalities caused by highway-wildlife interactions, though greatly skewed towards wildlife, also affect humans. The Wildlife Society estimates that 200 people are killed on the roads every year because of hitting an animal (4). All of these problems have lead to efforts to reduce the negative impacts of highways on wildlife and the human lives and money lost to animal-vehicle collisions (4). Specific strategies include wildlife bridges, underpasses, and sensors that trigger warning lights when animals come near the road (1). Large-scale approaches include integrating landscape ecology and conservation biology with regional transportation planning (4).

One such project is located in the Banff National Park and surrounding areas in Alberta, Canada (2). The park is crossed by the Trans-Canada Highway, and 22 wildlife underpasses, two overpasses, and fencing along the roads have been built and monitored in the last 25 years. Thanks to these efforts, ungulate road kill is estimated to have dropped by 96 percent (4). Monitoring track pads suggest that 10 large mammal species have used the crossings more than 70,000 times since the fall of 2005. A DNA/hair-sampling system was set up in 2004-2005 at two underpasses, using sticky wires stretched from end to end (2). The DNA collected provided information on genetic exchange for species such as bears, which usually have low densities and reproductive rates and are sensitive to fragmentation. Similar wildlife crossings have been built in other locations in North America, such as along Alligator Alley (I-75) in south Florida to provide for the wide range of the Florida panther, black bear, and other species (2). More research is needed, however, before it can be determined exactly how many species and to what extent these crossings are helping.

Human travel has affected the environment throughout history, often causing environmental degradation as a result (3). Today, humans are traveling more than ever, and the resulting impact on wildlife is severe. At the same time that we are beginning to understand what can be done to mitigate the effects of highways on wildlife, the U.S. is pursuing an aggressive transportation program across the country (5). Only if we prioritize minimizing destruction to wildlife when building highways will we be able to rein in the immense damage we are causing.

A wildlife overpass on the Trans-Canada Highway in Alberta's Banff National Park. (Source:


1. Ament, R. Wildlands Project: News You can Use: Critter Crossings. Retrieved March 27, 2008, from

2. Ament, R. and T. Clevenger. Wildlands Project: News You can Use: Lessons from Highway Wildlife Crossings in North Banff National Park. Retrieved March 27, 2008, from

3. Bui, T. Movement Throughout Human History and Its Effects. 26 March 2003. ENVS 02 webpage. Retrieved March 27, 2008, from

4. U.S. Department of Transportation Federal Highway Administration. Critter Crossings: Linking Habitats and Reducing Roadkill. Retrieved March 27, 2008 from

5. Transportation Research Board – The National Academies. (2002). Interaction Between Roadways and Wildlife Ecology: A Synthesis of Highway Practice ( National Cooperative Highway Research Program Synthesis 305). Available at:

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last updated 3/28/08