Mental_Floss has an awesome short article about Wildlife crossings, which doesn't just mean those signs that warn you that deer like to jump out around that part of the road. Wildlife crossings are these often incredibly beautiful pieces of architecture over highways that save wildlife the trouble of getting hit by a truck:
That and more pictures are available at The World Geography.
Wildlife crossings help all kinds of animals get around, including salamanders, panthers, bears, and badgers. These pieces of infrastructure save not just wildlife, but also money: Drivers in the U.S. spend $8 billion annually on wildlife-related damage to cars.
Wikipedia points out that these projects are trivial in cost, especially when compared to the gains, both environmentally and in property damage to people's cars:
The benefits derived from constructing wildlife crossings to extend wildlife migration corridors over and under major roads appear to outweigh the costs of construction and maintenance. One study estimates that adding wildlife crossings to a road project is only a 7-8% increase in the total cost of the project (Bank et al. 2002). Theoretically, the monetary costs associated with constructing and maintaining wildlife crossings in ecologically important areas are trumped by the benefits associated with protecting wildlife populations, reducing property damage to vehicles, and saving the lives of drivers and passengers by reducing the number of collisions caused by wildlife.
I talk a lot about liking the urban world more than the rural, but I'm not sure it always comes across that this is a big part of that -- finding ways to cooperate with, and share our spaces with, animals native to the sites of our development is hugely important.
And, it just generally makes the world a more pleasant place. Seeing a deer on the highway during a drive can go two ways: it can either be terrifying, because you're afraid it's going to jump out at you and you'll get into a horrible accident, or it can be pleasant, seeing a cute animal safely away from your car. Among everything else, these crossings sound like one of the millions of ways we can make lots of peoples' lives a little bit less shitty, which is how you make the world a nice place to live.