Practical Engineering - The Hidden Engineering of Wildlife Crossings

Short Summary

Main recommendation: Build wildlife crossings where they are most needed, choosing location and design by target species, because roads cause collisions, habitat fragmentation, and economic losses that wildlife crossings can reduce. The video explains that the best solution varies by species, so there is no one-size-fits-all crossing.

Key Points

• Wildlife crossings are costly; the Wallis Annenberg Wildlife Crossing over US-101 near Los Angeles is estimated at about $92 million and will be the largest on the planet when finished. The Federal Highway Administration set aside $350 million in federal funding for similar projects.
• Roads cause serious harm: the Federal Highway Administration reports over 1,000,000 wildlife-vehicle collisions annually in the US, with tens of thousands of injuries, about 200 human fatalities, and over $8 billion in damages per year. Many collisions go unreported.
• Economic losses justify the cost: researchers estimated the average deer collision in 2021 cost $9,100, and a moose collision over $40,000.
• Species respond to roads differently. Jacobsen's categories are non-responders (e.g., frogs), pausers (e.g., turtles), avoiders (e.g., grizzlies, cougars), and speeders (e.g., deer). Some species, like the cougar P-22, are exceptions.
• Crossing type depends on the species: underpasses and culverts suit small or moderate animals, while overpasses are often needed for large animals like moose.

Details Worth Noting

• Concrete instructions: Choose a crossing location using aerial photography, land cover, topography, habitat, ecological field data, and roadkill statistics. Design for a "focal species" and expect it to help others too. Place fencing to guide animals to the crossing; bury a few feet of fence where species like badgers or bears dig under; end fences in areas hard to cross, using boulders where useful; add ramps or "jumpouts" as one-way exits for animals trapped on the highway side; and add hinged doors for moderate-sized animals to escape through fences. For small amphibians, install many small culverts near one another with special fencing, and optionally buried collection buckets checked during peak migration, with animals manually carried across. For underpasses, maximize natural light, encourage natural vegetation, keep people out, and add cover objects like logs, rocks, and brush. Use canopy bridges for tree-dwelling animals such as primates and squirrels.
• Overpass bridges must hold their own weight with a significant safety margin, withstand decades of weather, be inspected like normal bridges, be covered in thick soil and vegetation (sometimes trees), and be wide enough for herds or wary animals. Walls can shield animals from traffic noise and light, and approaches can be steeper in mountainous areas.
• Overpass designs are usually beam bridges or arches with dirt instead of concrete for the deck, not cable-stayed or suspension designs, because the spans are usually not that long. Flared approaches help animals find and see the bridge.
• Roads near highways can attract scavengers and grazing animals, and cliff swallows nest under overpasses, sometimes called bridge swallows.
• Traffic affects crossings: at high traffic levels, some animals are "repelled" by the moving vehicles, which can make the impact harder to see and to fund.

Bottom Line

Wildlife crossings work best when they are matched to the target species' behavior, terrain, and traffic, because one design rarely serves all animals equally.