Designing Wildlife-Friendly Roadways

Javelinas utilizing constructed crossing under Twin Peaks Road.

By Alejandro Angel, PhD, PE, PTOE

Wildlife-friendly roadway crossing construction at Santa Rosa.

Wildlife-friendly roadway crossing construction at SR 86, Santa Rosa Ranch segment, in Arizona. Click all photos to enlarge.

Given the dense transportation network of roadways in the United States, conflicts between vehicles and wildlife are common. Highways and roadways create barriers to animal movements which results in fragmented habitats, disrupted gene flows and elevated wildlife mortality as animals attempt to cross them. 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 injures and more than $8 billion in property damage and medical costs in the United States alone.

Fortunately, biologists, engineers, political jurisdictions and the public have been working together to mitigate these issues. Psomas recently applied this multi-disciplinary process to three road projects that invested more than $7 million to avoid vehicle-wildlife conflicts. While Psomas was the lead designer, Arizona Game and Fish (AZGF) Department performed the wildlife mortality studies, the Regional Transportation Authority (RTA) provided funding, and the Arizona Department of Transportation, Pima County and the Towns of Marana and Oro Valley guided the design process.

The process usually starts by performing a wildlife mortality study. First, the species in the area are identified (in Arizona common species include mule deer, coyotes, javelinas, mountain lions and bobcats, snakes and desert tortoises, among others). Biologists then conduct field surveys of wildlife tracks and mortality. Finally, the information is analyzed to identify mortality hotspots for one or more species.

Using the hotspot data, engineers and biologists can work to effectively direct wildlife to specific road crossings through the use of wildlife fencing. 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 while placing the crossings in high wildlife activity areas (i.e., near the hotspots). 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-foot tall arch structures, and even one bridge.

Mountain Lion in a roadway crossing hotspot in Arizona. Photo credit: AZGF

Mountain Lion approaching a wildlife crossing at SR 86, Santa Rosa segment, Arizona. Photo credit: AZGF

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 wildlife use of the culvert. As examples, concrete culvert floors and riprap (rock-lined) basins are commonly used to prevent erosion, but reduce the use of the culvert by wildlife, which prefer a natural surface. Many animals also prefer to have line of sight to the end of the culvert in order to use the crossing; this may preclude the use of drop inlets that may otherwise be needed for drainage purposes. However, none of those issues are insurmountable. In our projects we have designed:

  • Special (gradual) drop inlets to provide animal line of sight
  • Culverts with baffle/silt treatments that help develop sand and small rock buildup over a concrete floor
  • Special outlet treatments with wildlife “sidewalks” on the periphery that allow the placement of riprap in areas requiring energy dissipation
  • Wildlife ramps to allow animals to negotiate grade drop-offs
Open-bottom arch structures under Twin Peaks Road accommodate drainage and provide safe wildlife crossings.

Open-bottom arch structures under Twin Peaks Road accommodate drainage and provide safe wildlife crossings. Javelinas in picture at top of page are utilizing this crossing.

Another issue is that the construction of miles of fencing adjacent to a road can often negatively impact the 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 or doesn’t work once a treatment is installed. On Twin Peaks Road, one of our recent projects, AZGF installed motion activated cameras to quantify activity at each crossing and conducted before and after wildlife mortality studies. AZGF found that many species use the crossings repeatedly, but that human activity at the crossings discourages use by certain species. Still, the project has been a huge success story. Wildlife mortality was reduced by over 90%, despite the fact that traffic along the road corridor tripled after construction of the project.

Alejandro Angel, PhD, PE, PTOE is vice president and director of Arizona Transportation and Traffic Engineering at Psomas.


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