Roads help connect our world, however without carefully thought through plans a road can split one endangered but viable population of a species into 2 unconnected populations that are likely to disappear over the long term.
Genetic studies have shown that only one bear has managed to cross between these two Spanish bear populations. While there are only 3 major roads, there are plenty of large areas unfriendly to bears crossing through. What should put this in context, is that one bear has also managed to cross France unaided to join this population.
Successful bridge connectors have been used in various countries around the world. Sweden has a series of bridges to allow reindeer to cross the roads safely and to stop this delaying traffic. Christmas island has created bridges that allow the crabs to complete their migration. Around LA a series of bridges have been created to connect the small areas left for mountain lions in the area.
In the long term these bridges often save money, though perhaps not by the builders. Without a bridge across, most species will instead cross the road itself. This can lead to them being killed on the road, as well as significant cost of repairing the cars, or indeed the death of the occupants.
Animal crossing bridges tend to need to be different to other bridges if they are to be successful. Generally, the bridge must be wider, it must also have vegetation on it to give animals cover and avoid the feeling of being out in the open. Having said this, given the lack of any need for lighting or regular resurfacing, these crossings can be cheap to maintain.
We need to move in the direction, where if any road or railway is being built, means for wildlife to cross safely is also included – underpasses are also possible, though as these often appear unnatural they often have lower levels of success. We must also go back to many of these old transport links and add wildlife crossings so that we reconnect populations. There are many subspecies of animal that could be saved from extinction by simple moves like this.