Designing Wildlife-Friendly Roadways

Designing Wildlife-Friendly Roadways: Twin Peaks crossing

Open-bottom arch structures under Twin Peaks Road accommodate drainage and provide safe wildlife crossings.

Multidisciplinary process includes mortality studies, crossing designs, and fence selection.

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As published by civil + structural ENGINEER magazine  |  Abridged version published by ENR

By Alejandro Angel, PhD, PE, PTOE, ENV SP

Given the dense 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.

Awareness about this issue has been growing, and several states have invested significant funds to address the problem. Arizona has constructed elk crosswalks as well as multiple wildlife underpasses, and is currently building an $11 million wildlife bridge across a busy state route. Other examples include the State of Washington, which recently broke ground on its first freeway overpass for animals, and California, where plans have just been announced to build a landscaped, 165-foot-wide, 200-foot-long wildlife overpass over the 101 Freeway in Los Angeles.

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. While Psomas was the lead designer on these projects, the Arizona Game and Fish Department performed the wildlife mortality studies, the Regional Transportation Authority provided funding, and the Arizona Department of Transportation, Pima County, and the Towns of Marana and Oro Valley guided the design process.

Mountain Lion approaching a wildlife crossing at SR 86, Santa Rosa segment, Arizona.

Mountain Lion approaching a wildlife crossing at SR 86, Santa Rosa segment, Arizona.

Identifying and Selecting Wildlife Crossing Locations

The process usually starts by performing a wildlife mortality study. The first step in the study is to identify the species in the area (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 wildlife mortality. Finally, the track and mortality data are entered into GIS and analyzed by segmenting the project into cells of equal size. Cells with higher wildlife-vehicle collision frequency are referred to as “hotspots.” The resulting study documents locations with mortality hotspots for one or more species.

The next step in the process is to determine how to mitigate the hotspots identified in the study. Generally, there are two options available: provide a wildlife crossing, or prevent crossing at that location by installing fencing and forcing animals to find a different crossing location. Redirecting wildlife to another location may be appropriate if a crossing would not be viable in the long term due to encroaching development or other related factors. Funding constraints also play a role on how many crossings can be provided on a given project. On a recent roadway project, wildlife crossing locations were prioritized using the following criteria:

  • Identification of the crossing on a local or federal conservation plan or set-aside conservation land system.
  • Types of adjacent land uses (existing and planned) — Lower intensity uses generally preserve the area around the crossing in a more natural state.
  • Quality of adjacent vegetation.
  • Level of connectivity upstream and downstream along the wildlife corridor (typically a creek) — As an example, it’s inefficient to build a large structure for wildlife if the next downstream crossing is already developed as a smaller structure or pipes.
  • Light pollution level (existing and planned) — Many species shy away from lit areas. Therefore, nearby traffic signals, parking lot lighting, and other similar sources may make a crossing less desirable.
  • Impact of a potential crossing on roadway design — This includes evaluating both safety (such as stopping and intersection sight distance) and cost (earthwork volume, cost of structure).

On our Tangerine Road project, we ranked all hotspot locations using the criteria above and, based on the available moneys, decided to pursue crossings at the top eight ranked locations (out of 13 hotspots). At the remaining hotspots, we provided fencing to direct animals to the corridors that included wildlife crossings.

Designing Wildlife-Friendly Roadways: Javelinas crossing

The Arizona Game and Fish Department installed motion-activated cameras to quantify activity at each crossing, such as these javelinas.

Design of Wildlife Crossing Treatments

As previously discussed, a comprehensive wildlife crossing treatment includes two primary elements: crossing structures and fencing to direct animals to those structures. The target species for the area determine many of the design elements for both the crossings and fencing.

Wildlife crossings are often provided by oversizing drainage structures. Because they support more vegetation and animal life, creeks and washes (dry riverbeds) usually have high levels of wildlife activities that coincide with the hotspots. Locating the wildlife structures at drainage crossings also helps contain costs.

Target species also determine the height, width, surface, and other key features of the crossings. The size of a crossing needed by a target species is usually expressed in terms of two parameters: a minimum height and a minimum “Openness Index” (OI). The OI is the ratio of the cross-sectional area of the structure (width x height) to the length of the structure, with all the dimensions in meters. Do not perform these calculations in feet because it would result in undersized crossings.

Based on previous projects, we have learned that medium-sized mammals in Arizona (coyotes, javelinas, mountain lions) generally accept structures of 6 feet in height and a minimum OI of 0.25, which for a typical four-lane highway results in structures 6 feet high and 20 feet wide (assuming 150-foot length). Structures for large mammals (mule deer) require 9 feet in height and a minimum OI of 0.75, which under the same conditions translates to structures 9 feet high and 40 feet wide. On recent projects, crossings have ranged in size from small pipes (for reptiles, amphibians, and small mammals) to 9-foot-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 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 be able to spot predators; this may preclude the use of drop inlets that may otherwise be needed for drainage purposes. However, none of those issues are insurmountable. Psomas has incorporated the following elements to address such issues:

  • 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, and
  • wildlife ramps to allow animals to negotiate grade drop-offs.
Concrete arch structures can provide sufficient height and width for large mammals.

Concrete arch structures can provide sufficient height and width for large mammals.

Wildlife fencing typically serves a wide range of target species. As a result, fences often combine both fine and large size mesh. The fence is typically buried to avoid having animals dig under it, the bottom two to three feet use a fine (1/4-inch) mesh to keep smaller wildlife from going through the opening, and the rest of the fence (to a height of approximately 8 feet if designed for deer) consists of a mesh with 6-inch openings.

Another issue is that the construction of miles of fencing adjacent to a road can negatively impact the visual environment for residents and road users. Ideally, the fence should be placed as far from the roadway as possible. However, sometimes this results in the fine mesh of the wildlife fence blocking roadway drainage or preventing offsite drainage to reach the conveyance channels.

To solve this issue, in the past we have placed the fence in locations more conducive to drainage (typically closer to the roadway), but coated the fence in colors that blend with the background. We are also currently 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.

Evaluating the Benefits

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, the Arizona Game and Fish (AZGF) Department 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 more than 90 percent, despite the fact that traffic along the road corridor tripled after construction of the project.


Alejandro Angel, PhD, PE, PTOE, ENV SP, a principal, vice president, and member of the board of directors of Psomas, oversees the firm’s engineering teams in Arizona and San Diego. He is also a member of the Arizona Board of Technical Registration and was recognized by Civil + Structural Engineer magazine as a Rising Star in Civil Engineering in 2015.

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