Roadways fragment natural habitat and can have significant impacts on wildlife movement and survival. Effects of roads on wildlife include direct mortality from animals killed by vehicle collisions, the formation of barriers to animal movement, potential restriction of gene flow, and road avoidance due to noise and traffic. Wildlife-vehicle collisions are also a serious safety problem for humans.

Drainage culverts, tunnels, roadway underpasses, and roadway overpasses can be effective tools in reducing road mortalities and increasing connectivity in fragmented landscapes. The NPS and CalTrans have been working together to reduce wildlife mortality along and connect natural habitat occurring on both sides of several Southern California highways in and around Santa Monica Mountains National Recreation Area. These efforts include clearing out underpasses under the highway, constructing effective fences along both sides of the highway, and installing one-way gates to allow animals that are inside the fencing to escape. To measure the effectiveness of these efforts, the NPS has monitored culvert use by wildlife and road mortality of wildlife before and after highway modifications. Measuring these different aspects of wildlife movement and mortality both before and after modifications occur provides an excellent opportunity to evaluate the efficacy of the mitigation. These results will have important implications for future efforts to mitigate the impacts of roads and vehicles on the movement and survival of wildlife.