The elk in Banff, Alberta were photographed crossing a freeway exit during rush hour. 

Transportation corridor designers have been challenged to reduce the collisions.  A solution is to build grade separation structures for the animals.  These structures are usually underpasses, although bridges can work as shown by the first photo.

In addition to more mundane structural issues, critter crossings have some special design concerns.  The passageways must be designed in such a way as to seem (relatively) natural for their users.  Therefore, approaches to the underpasses should have appropriate vegetation and detailing.  In many cases, it is important to avoid signs of human use and contact, since this would tend to scare away many species from using the crossings.  This is why it is usually not desirable to combine a critter crossing with a pedestrian trail.  When human pedestrians use the trail, many animals will be scared away by the scent.

Another complication is that while facilities for humans are typically designed for only one species, critter crossing designs must account for multiple species.  The various quirks of different animals must be considered.  For example, carnivores and herbivores probably will not congregate so easily at a particular critter crossing.  The carnivores would like to, since a new underpass that attracts their prey would serve as diners for them.  But their prey would quickly learn to avoid the sites.

Wildlife tunnels shouldn’t not be too dark for too long, leading to the need for placement of  natural skylights.  For a freeway underpass, this may mean opening up space in the median. 

Although there are general guidelines for critter crossings, the designs are very specialized and site-specific.  The designs are impacted by the location, transportation facility, and the critters of concern.  An FHWA website anecdotally describes some of the differences.  Underpasses for desert tortoises have design requirements much different than those for wolves, for example.

The need for critter crossings itself is part of an ongoing evolution of design requirements for civil engineers.  Up to fifty years ago, the mission was to develop, progress and tame the frontier.  There was not much understanding that we were part of the frontier.  The new, evolving paradigm is for sustainability and for our infrastructure to better harmonize with the environment.  At times this shift requires a difficult adjustment, such as when we need to weigh alternatives that somehow balance long term goals with the more familiar short-term cost/benefit evaluations.  Our current tool set still tends to treat these as apples and oranges.  But the paradigm change is necessary and ultimately welcome.  As an added benefit, when we successfully build critter crossings in our projects, in addition to helping the animals, we reduce the odds of personally ending up as road kill.