December 6, 2018

Why did the chicken cross the road? Yes, to get to the other side, however I’m betting he didn’t have the convenience of a wildlife crossing. The Federal Highway Administration estimates that there are between one and two million collisions between wildlife and vehicles every year. Not only does this present a danger to wildlife survival, but it also poses a real threat to human safety. Additionally, as urban sprawl continues and roadways encroach on animal habitats, native environments become fragmented and wildlife struggle to survive.

The American Association of State Highway and Transportation Officials (AASHTO) has provisions for vehicle and pedestrian bridges, but there’s not as much available for designing crossings for wildlife. By designing designated wildlife crossings we can meet the needs of the traveling public, maintain human safety and conserve wildlife. While animals can’t read street signs, they’re not immune to the psychological influences of intentionally engineered structures.

Our Transportation team in Florida has been selected to design a new pedestrian trail and wildlife crossing in Jonathan Dickinson State Park. This crossing will allow wildlife to cross a very active rail line where a new high-speed train will soon begin regular service. Keep reading to learn about the design considerations we’re taking into account for guiding wildlife in the right direction.



Know your Audience

Understanding who, or rather what, will use the crossing is more than half the battle. Some animals, such as rodents, prefer covered, enclosed spaces, while deer and panthers prefer large expanses where they can see what’s ahead of them. In general, the more a wildlife crossing resembles an animal’s natural habitat, the more effective it will be.

Additionally, the crossing’s structural needs will differ based on how much load it will support. Whether it’s beach mice or black bears makes a difference! It’s also important to consider if mechanized vehicles will be using the structure. If so, consider how much will it weigh and if the size will affect the bridge’s components or the way wildlife use the crossing.

A major consideration when incorporating soil on a structure is drainage. Isolating and moving water from the structure is important from a loading perspective, as well as from a durability perspective. Saturated soil combined with potential vehicular loads creates a load combination that’s different than a typical highway bridge.



Location, Location, Location

The real estate mantra still rings true: it’s all about location, location, location. Similar to buying the right home in the wrong location, the best wildlife crossing in the wrong spot won’t be adequately used by animals. Many animals move along the same corridors year after year, while some migration patterns are more random. Using data from wildlife cameras and collared animals to assess movement patterns and inform the design process is key in designing a crossing that gets used.

Also, consider if there’s a nearby greenway or trail for pedestrians and bikers. If there is, what considerations should be taken to lessen human distractions for wildlife? Traffic noise, light from headlights and human activity can reduce the effectiveness of a crossing, however crossings designed for both human and animal use do exist and may be an option in certain locations.



Keep it in Context

It’s important to have a detailed, thorough understanding of the crossing’s endstate, or the environment at the destination. By understanding what type of vegetation is present and the site’s ability to sustain certain types of native plant material, we can make informed design decisions that will promote the bridge’s effectiveness and overall longevity. Additionally, appropriate vegetation on and around the crossing helps recreate the original habitat and lures animals across the structure with scents.

Providing wet-ponds at each of the crossing’s landings can also help bring wildlife to the crossing, with the presence of fresh water giving animals a reason to use the structure.  Wet-ponds also provide refuge for amphibians that may need the water both before and after their crossing.

While keeping remote areas roadless is a good idea, it’s not always a viable option. By building wildlife crossings that take the user, location and context into account, we can reconnect fragmented habitats and reduce vehicle-animal collisions—helping the proverbial chicken get to the other side safely and stress-free.