Generically these are known as Animals Crossings as any fabricated structure, which facilitates the movements of animals across an impeding obstacle such as roads, or railway line could serve as a crossing.
However, as the most spectacular crossings are bridges, the focus will be on bridges.
Main picture: A Green Wildlife Bridge over an Autobahn in Germany
Even though the primary focus of conservation efforts are on the preservation of their habitats and their numbers, a lesser factor in these efforts is to prevent the fragmentation of the habitat into unsustainable pieces of land.
Every additional highway or railway line has the unintended consequence of fragmenting the natural habitat penning animals into an ever-diminishing area. The inexorably consequence is species gene pool being shrunken. In addition, the natural range of many animals is vast.
In order to overcome all these negative consequences, many countries with potent wildlife conservation ethics, have since the 1950s been constructing animal bridges.
Obviously, a subsidiary benefit of these bridges is the reduction of animal/car collisions.