Pronghorn and Mule Deer Use of Underpasses and Overpasses Along US Highway 191, Wyoming

The seasonal migrations of ungulates are increasingly threatened by various forms of anthropogenic disturbance, including roads, fences, and other infrastructure. While roadway impacts (i.e., wildlife‐vehicle collisions and landscape permeability) of two‐lane highways to mule deer (Odocoileus hemionus) can largely be mitigated with underpasses and continuous fencing, similar mitigation may not be effective for pronghorn (Antilocapra americana) or other ungulate species that are reluctant to move through confined areas. The Wyoming Department of Transportation recently installed 6 underpasses and 2 overpasses along 20 km of US Highway 191 in western Wyoming, where the project team evaluated species‐specific preferences by documenting the number of migratory mule deer and pronghorn that used adjacent overpass and underpasses for 3 years following construction. The project team also measured the amount of back and forth movement across the highway for each species through time. The team documented 40,251 mule deer and 19,290 pronghorn migrate across the highway. Of those, 79% of mule deer moved under, whereas 93% of pronghorn moved over the highway. These strong species‐specific differences were evident at both sites and support the notion that overpasses are more amenable to pronghorn than underpasses. Concurrently, the project team documented a sharp increase in the amount of back and forth movement of mule deer and pronghorn across the highway during migration periods. Such movement flexibility is presumed to improve their ability to respond to changing environmental conditions by easily accessing habitats on either side of the highway. The project team's results highlight that species‐specific preferences are an important consideration when mitigating roadway impacts with wildlife crossing structures. Overpass and underpass construction reduced wildlife‐vehicle collisions by approximately 81%.