Enhancing Existing Isolated Underpasses with Fencing Reduces Wildlife Crashes and Connects Habitat

The impact of wildlife–vehicle collisions on drivers and wildlife populations has been gaining attention in the United States. Given the established success of wildlife crossings with fencing in reducing wildlife crashes and connecting habitat, information is needed on cost-effective means of implementation for departments of transportation. When wildlife crossings are constructed, they are often built into new road projects as a series of 2 or more underpasses and/or overpass structures connected by exclusionary fencing. Given limited transportation budgets and the prevalence of maintenance activities more so than new construction in many states, enhancing existing underpasses on previously constructed roads has been recognized as a cost-effective mitigation opportunity. More research is needed, however, on the effects of adding fencing to existing underpasses, particularly those that are too far from one another to be connected with contiguous fencing. In this study, we evaluated the effectiveness of this measure when applied to isolated underpasses. Approximately 1.6 km of 2.4-m-high wildlife fencing was added to each of 2 existing underpasses, a large bridge underpass and a large box culvert, situated approximately 8 km apart from one another on Interstate 64 in Virginia, USA. We conducted a 2-year post-fencing camera monitoring study and compared the findings from a 2-year pre-fencing study with regard to collision frequencies with white-tailed deer (Odocoileus virginianus) and black bears (Ursus americanus); the use of the underpasses by wildlife; and roadside deer activity. We also evaluated deer activity data to compare different fence end designs applied at the study sites. After fencing installation, deer–vehicle collisions (DVCs) were reduced by 96.5% and 88% at the box culvert and bridge underpass, respectively, and there were no increases in DVCs within 1.6 km of the fence ends. Deer crossings increased 410% at the box culvert and 71% at the bridge underpass. Use of the culvert and bridge underpasses by other mammals increased 81% and 165%, respectively. Although deer use of the underpasses was much greater than their activity at any of the fence ends, there was relatively high deer activity at the fence ends that did not tie into a feature such as right-of-way fencing. Our study found that the addition of wildlife fencing to certain existing isolated underpasses can be a highly cost-effective means of increasing driver safety and enhancing habitat connectivity for wildlife. The benefits from crash reduction exceeded the fencing costs in 1.8 years, and fencing resulted in an average saving of >$2.3 million per site over the 25-year lifetime of the fencing. The results add to the growing body of knowledge about effective ways we can use existing infrastructure to connect wildlife habitat and increase driver safety.