Given the dense transportation network of roadways in the U.S., conflicts between vehicles and wildlife are common.
Highways and roadways create barriers to animal movements, which result in fragmented habitats, disrupted gene flows and elevated wildlife mortality as animals attempt to cross. Many endangered species are threatened by roadway mortality.
But the problem is not limited to wildlife. Every year, vehicle-wildlife collisions are responsible for 200 human deaths, 2,600 injuries and more than $8 billion in property damage and medical costs in the U.S. alone.
As awareness of this issue has grown, biologists, engineers, political jurisdictions and the public have been working together to mitigate these issues. Psomas recently applied this multidisciplinary process to four road projects that invested more than $10 million to avoid vehicle-wildlife conflicts in Arizona.
The process usually starts by performing a wildlife mortality study. Biologists then conduct field surveys of wildlife tracks and mortality. Finally, the information is analyzed to identify “mortality hot spots” for one or more species.
Using the hot spots data, engineers and biologists can work effectively to direct wildlife to specific road crossings using wildlife fencing. The target species for the area determine the height and opening size of the fence to be used. Oversized drainage structures are commonly used to provide wildlife crossings.
This helps contain costs by placing the crossings only in high wildlife-activity areas (i.e. near the hot spots). Again, the target species also determine the height, width, surface and other key features of the crossings. On our recent projects, the crossings have ranged in size from small pipes to 9-ft-tall arch structures and even one bridge.
Designing structures to serve both as drainage and wildlife crossings is often challenging because what may work best for drainage may not be best for the wildlife. For example, concrete culvert floors and riprap (rock-lined) basins are commonly used to prevent erosion, but they also reduce the use of the culvert by most wildlife, which prefers a natural surface.
Many animals also prefer to have line of sight to the end of the culvert in order to spot predators. This may preclude the use of drop inlets that otherwise may be needed for drainage purposes.
However, none of these issues are insurmountable. In the projects we have designed:
- Special (gradual) drop inlets provide animals with line of sight.
- Culverts with baffle/silt treatments help develop sand and small-rock buildup over a concrete floor.
- Special outlet treatments with wildlife “sidewalks” on the periphery allow the placement of riprap in areas requiring energy dissipation.
- Wildlife ramps allow animals to negotiate grade drop-offs.
Another issue is that construction of miles of fencing adjacent to a road can often lessen visual aesthetics for residents and road users.
We are experimenting with the use of “invisible fence,” a mesh of very thin cables developed in Tucson to better blend with the background and mitigate visual impacts.
The science behind wildlife crossings is a rapidly developing field, and it is important to evaluate what works and what doesn’t once a treatment is installed.