Surface transportation cuts across natural habitats vital to species survival and ecological biodiversity. As road density increases, so do edge habitats, fragmenting the remaining wildlife habitat into smaller and smaller pieces while increasing the area where vehicle-wildlife collisions are likely to occur. As the Federal Highway Administration’s (FHWA) Wildlife Crossing Structure Handbook notes, fragmentation is particularly harmful to “wildlife that have large area needs, are found in relatively low densities, and have low reproductive rates.” When road infrastructure separates wildlife populations from vital sources of food, water, or shelter; or where roads bisect optimal habitat, they can be especially harmful to wildlife survival. But road infrastructure can incorporate crossings to keep habitats connected even as roads grow and develop—integrating, rather than dividing, the habitats.

Habitat fragmentation both reduces the area of the habitat available and changes the qualities of that habitat: Habitat fragmentation is an aspect of habitat configuration. Connecting habitats on two sides of a divider, such as a highway, wildlife crossings reconfigure habitats to better support ecologies on both sides of the divide. The smaller a habitat patch, the fewer species it can sustain. Still, fragmentation, when controlling for habitat loss in general, may have some positive effects on species that benefit from immigration rate, patch isolation, access to multiple habitats (for animals that spend their adult lives in a different habitat from where they breed or birth), and positive edge effects, especially those species whose main predators need wide-open ranges for survival. The effects of fragmentation or connectivity on a particular species depend on that species’ needs.

Even absent intentional design, wildlife already cross public roads. The 2004 Biological Conservation study “Use of Highway Undercrossings by Wildlife in Southern California” reported “regular use of underpasses and drainage culverts beneath highways by wildlife, including species of conservation concern.” The Biological Conservation study authors recommended building pathways intended primarily to support crossings that connect suitable habitats “for protecting native species in areas bisected by high-speed roadways.”

However, the article “Impact of Wildlife Crossing Structures on Wildlife-Vehicle Collisions,” published in the Transportation Research Record in 2004, concluded: “collision reductions were more consistent among wildlife bridges than culverts” and estimated the annual financial benefits from reduced wildlife-vehicle collisions for each crossing structure to be between $235,000–$443,000. In a 2007 report to Congress, Wildlife-Vehicle Collision Reduction Study, Hujiser et al. estimated the cost of a wildlife overcrossing at $3.5 million, which, in light of their assumption of an 80-year useful life and the Transportation Research Record study’s estimation of benefits, may indicate substantial net savings from reducing wildlife-vehicle collisions through properly placed overcrossings. In particular, wildlife overcrossings, such as landscape bridges and wildlife overpasses, are more suitable for large mammals that may not be willing to regularly use underpasses and culverts.

One such animal is the bighorn sheep, occupying the deserts and mountains of the southwestern United States. Traffic is one of the leading killers of bighorn sheep, which depend on a wide-ranging habitat. Caltrans (California’s transportation agency), California Department of Fish and Wildlife, and a private rail company have agreed to develop three wildlife crossings above I–15 between Las Vegas and southern California. These overcrossings would span the width of the current I–15 highway as well as the proposed rail system in the median. The highway borders the Mojave National Preserve and passes several wilderness areas home to bighorn sheep, bisecting their natural habitat.

FHWA’s report to Congress estimated there are between one and two million wildlife-vehicle collisions per year, and large mammals such as moose and deer account for many of those collisions causing human injury or fatality. In considering that report, Congress created the Wildlife Crossings Pilot Program to provide competitive grants for improving habitat connectivity and reducing collisions. Wildlife crossings such as the three proposed over I–15 are one measure transportation planners can use to address the greater than $8 billion annual cost of wildlife-vehicle collisions to improve driver safety and preserve biodiversity that occur around the Nation. Public roads need not be at loggerheads with the bighorn sheep and other large mammals; by integrating roadways with existing ecologies, overcrossings can reduce traffic fatalities and provide substantial savings for drivers.

Joseph Parampathu is a sophomore at Allen Community College in Iola, KS. Joseph is currently studying accounting and will graduate in 2024.