Enhancing Existing Isolated Underpasses With Fencing to Decrease Wildlife Crashes and Increase Habitat Connectivity

About the report:

The impact of wildlife-vehicle collisions on drivers and wildlife populations has been gaining increasing attention in the United States.  Given the established success of wildlife crossings with fencing in reducing wildlife crashes and connecting habitat, a growing number of states, including Virginia, have enacted wildlife corridor legislation, some of which encourages or requires the construction of wildlife crossings along identified wildlife corridors and/or high-crash areas.  Because of the growing interest in wildlife crossing measures, research is needed on cost-effective means of implementation for departments of transportation. 

When wildlife crossings are constructed, they are often built into new road construction projects as a series of two or more underpasses and/or overpass structures connected by exclusionary fencing.  Given limited transportation budgets, enhancing existing underpasses on previously constructed roads has also 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.  

The purpose of this study was to determine the effectiveness of enhancing existing isolated underpasses with wildlife fencing.  One mile of 8-ft-high wildlife fencing was added to a large bridge underpass and a large box culvert 5 mi apart on Virginia’s I-64. Effectiveness was determined by conducting a 2-year post-fencing camera monitoring study and comparing the findings with those from a 2-year pre-fencing study with regard to the frequency of deer-vehicle collisions (DVCs); the use of the underpasses by deer and other wildlife; and roadside deer activity.  The study also used deer behavior and activity data to make comparisons among different fence end designs and jump out designs applied at the study sites.

The 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.  After fencing installation, DVCs were reduced by 92% on average (96.5% and 88% at the box culvert and bridge underpass, respectively). 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.  DVCs did not increase at the fence ends, but there was high deer activity at the ends that did not tie into a feature such as right-of-way fencing.   

At the study sites, the benefits from crash reduction exceeded the fencing costs in 1.8 years, and fencing resulted in an average savings of more than $2.3 million per site.  The findings from this study should be considered when DVC mitigation and/or wildlife connectivity measures are needed.  Wildlife crossing and fencing guidelines will be developed to provide the Virginia Department of Transportation with a resource for the cost-effective implementation of this wildlife crash mitigation measure.

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