When we design things like this in reclamation, there is often the concern that the designed feature can act like a trap or increase predation. There was an article a while back that showed that these are actually safe, and blend into the surrounding landscape in terms of risk to the animals.
Oh look, someone actually backing their viewpoint up on the internet:
i'd imagine that risk would be easily outweighed by animals being able to safely cross anyways, not like predators just sit there inhaling prey unceasingly, they would go there to catch something then leave with their food to go eat.
While you're right, in that predators won't sit there, doing their best to get on Season 8 of My 600-lb life, they definitely exploit linear corridors like this if they aren't properly constructed. Transmission lines, for instance, can be really gnarly for ungulates. Both predators and prey are smart. For predators, if there's an easy meal to be had, they'll continue to over-use this feature on the landscape, rather than the rest of their habitat (why cook, when you can order in?). This in and of itself is an impact on the natural ecosystem, and something we try to avoid, when the goal is 'no impact' or 'as close to no impact as possible'.
From a prey standpoint, if you see Jim-Bob getting ripped apart by wolves, or got chased by pack of hungry wolves, you'd be pretty hesitant to use that feature again, now wouldn't you? In this regard, not properly designing these things can essentially render them useless to ungulates and the like, and prevent the re-connection of the two polygons (areas) we're trying to connect.
I sent it better to funnel wildlife into traps for predators than to have the wildlife killed by vehicles on a road, attracting those predators onto the road seeking out their carcasses only to be hit by vehicles themselves?
Many owls and raptors get killed because they are trying to feed on a carcass in the road and get hit themselves.
With these corridors, they fence the road so that the animals can only pass over the bridge. And youre right, sometimes they do raise the road and let animals pass under. It depends on the topography.