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Wildlife underpasses include bridges, culverts, pipes, and tunnels and their dimensions vary substantially. Underpasses allow wildlife to cross safely to the other side of the road. The bridges are for the traffic; the animals walk under the road, and the vehicles go over the structure. Structures intended for large mammals are often at least 7 m (21 ft) wide and and 4-5 m (12-15 ft) high. However, if roads cross floodplains or of there are other factors involved (e.g. threatened or endangered species) bridges may be several hundred meters (several hundreds of yards) or several kilometers (several miles) long. Culverts or box culverts intended for medium sized mammals such as bobcats or coyotes may be 1.2-1.5 m (4-5 ft) in diameter. Smaller structures or pipes (e.g. 0.3-0.6 m (1-2 ft)) in diameter may be used by amphibians, reptiles, small mammals, and also by some medium sized mammals. It is desirable for underpasses to have soil, litter, and cover, but the lack of light and moisture may prevent vegetation from growing. Cover can be provided by tree stumps, branches, rocks, or other materials. The openness of a structure is important; it is best if the animals can easily see the vegetation or sky on the other side of the structure when they approach. Amphibians have particular requirements for soil and air humidity. Therefore amphibian tunnels need to have soil on the bottom and an open top (e.g. grating). Some underpasses have water flow through them, but such stream or river crossings should preferably be wide enough to allow for more than just water to flow under the road. Ideally stream or river crossings are wide enough to allow for natural stream dynamics so that the water flows just like it does upstream and downstream, with similar substrate and without an increase in velocity. The latter not only leads to erosion but can also form a barrier for many aquatic species. The underpasses should be wide enough to allow for riparian as well as dry zones to accommodate semi-aquatic and terrestrial species. Wider stream or river crossings also allow for some changes in the channel (natural meandering and stream dynamics), and have reduced maintenance and failure issues (less erosion, less plugging up with debris). Combined with wildlife fencing and overpasses, wildlife underpasses can reduce collisions with large mammals by 80 to almost 100%. Numerous studies have also found that a wide range of wildlife species uses wildlife underpasses, often in high numbers. This is important as animals may need to reach the other side of the road for food or water. Crossings by certain individuals maybe almost daily as they have their home range on either side of the road. Other individuals may use crossing structures for seasonal migration or long distance dispersal. Migration over long distances is important as animals may need to reach habitat patches that are small and isolated so that they can strengthen their population viability in the region.

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