Comparing the Two Engineering Models Driving North America's Wildlife Crossing Boom

The push to reconnect North America's fragmented wilderness has fractured into two distinct engineering camps. On one side, landscape architects and biodiversity advocates argue for wide, vegetated overpasses that mimic the natural environment, insisting that wary apex predators and herd animals will only cross if they cannot see the highway below. On the other side, transportation economists and state highway departments point to corrugated steel underpasses and concrete culverts, arguing that building cheaper, narrower tunnels allows agencies to protect far more miles of roadway for the same budget.[3]

The stakes for resolving this structural debate are measured in both human safety and ecological survival. The economic toll of a single severe crash is staggering: the Washington State Department of Transportation estimates that an average vehicle-deer collision costs $9,175, while hitting an elk balloons the cost to $24,242, and a moose collision reaches $42,652. To mitigate these losses, federal and state agencies are deploying unprecedented capital, but how those funds are allocated depends entirely on which engineering model a state adopts.[1]

The vegetated overpass model prioritizes ecological fidelity. These structures, which span multiple lanes of traffic, are covered in native soil, planted with local flora, and designed to look indistinguishable from the surrounding forest. This width and openness are critical for species that are sensitive to confinement, such as grizzly bears, pronghorn, and large herds of elk, which often refuse to enter enclosed spaces where they cannot see a clear exit.[2]

The drawback of the overpass model is its immense capital requirement. A standard wildlife overpass can cost between $2.7 million and $6.2 million. Because they are so expensive, transportation departments can only afford to build them at the most critical migration bottlenecks. In Utah, for example, the state has constructed 119 wildlife crossing structures to date, but only two of them are overpasses.[1][2]

Conversely, the underpass model relies on corrugated steel culverts and concrete tunnels bored beneath the road surface. These structures are significantly cheaper, ranging from $500,000 to $2.7 million to construct. Because they require less material and can often be integrated into existing drainage projects, highway departments can deploy them in higher volumes.[1]

The financial math heavily favors the underpass. Comparing the high-end construction costs of both structures against the $24,242 economic cost of an elk collision reveals a stark divergence in payback periods. An underpass reaches its economic break-even point after preventing approximately 111 elk collisions. An overpass, meanwhile, requires preventing 255 collisions to pay for itself. For a state transportation department operating on a fixed budget, underpasses deliver a faster return on investment.[1][3]

However, underpasses fail to accommodate the full spectrum of North American wildlife. Prey species rely on long sightlines to detect predators and are notoriously hesitant to enter dark, narrow tunnels. If an underpass is too long or too narrow, it effectively acts as a concrete barrier for these animals, severing their migration routes just as permanently as the highway itself.[2][3]

The most comprehensive data on how these two models perform side-by-side comes from Canada's Banff National Park. Over the past three decades, Parks Canada has constructed 44 wildlife crossing structures—six overpasses and 38 underpasses—along an 82-kilometer stretch of the Trans-Canada Highway.

Paired with extensive exclusion fencing, this hybrid network has reduced wildlife-vehicle collisions by more than 80 percent overall, and by 96 percent for elk and deer. Banff's monitoring program demonstrates that different species self-segregate by structure type, with wary species taking up to five years to adapt to the crossings.

Ultimately, the decision between an overpass and an underpass is a calculation of cost-efficiency versus ecological completeness. While underpasses deliver a faster financial return on investment by preventing common deer collisions at a lower upfront cost, overpasses remain the only proven mechanism for sustaining the genetic diversity of apex predators and large migratory herds across North America's busiest corridors.[2][3]