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ARoads cause problems for wildlife in two main ways. The most visible is collisions, which kill animals and can injure or kill drivers as well. Less visible, but just as important, is the barrier effect: a busy road can divide a habitat so that animals cannot reach food, water or mates on the other side. Over time, populations cut off in this way may become smaller and less genetically varied. Wildlife crossings, which are bridges or tunnels built for animals, aim to reduce both problems.

BBanff National Park in Canada has one of the best-known networks. Along the Trans-Canada Highway, which runs through the park, there are 38 underpasses and 6 overpasses, built in stages between 1981 and 2014. The crossings are combined with 82 kilometres of fencing, which keeps animals off the road and guides them towards the places where they can cross safely. To decide where the structures should go, researchers studied animal movements using methods such as radio tracking, snow tracking and records of animals killed on the road.

CMonitoring at Banff has continued since 1996, and more than 150,000 wildlife crossings have been recorded. According to Parks Canada, collisions between vehicles and wildlife have fallen by more than 80 per cent, and by over 96 per cent for elk and deer. Not all species prefer the same design: grizzly bears, wolves, elk and moose favour high, wide and short structures, whereas black bears and cougars more often use longer, lower underpasses. Species also differed in how quickly they adapted. Elk used the crossings early, even during construction, but cautious grizzly bears took up to five years.

DThe idea is now spreading to more crowded places. In California, construction began in 2022 on the Wallis Annenberg Wildlife Crossing, a bridge over the US-101 freeway at Liberty Canyon, near Los Angeles, designed to be covered in vegetation. The freeway has long separated animals such as mountain lions, bobcats, coyotes and mule deer that live in the mountain ranges on either side. The project design includes native plants on the bridge and sound walls intended to shield animals from traffic noise and light.