In 2015 and 2016, Colorado’s first two wildlife overpasses, five wildlife underpasses, 10 miles of wildlife fencing, and other mitigation features were constructed on State Highway 9 (SH 9) to address the issues wildlife-vehicle collisions (WVC) while maintaining permeability for wildlife. This project was the result of private partners joining forces with the transportation agency to advance, and ultimately fund the mitigation effort. Both the privately owned Blue Valley Ranch and the community-driven group, Citizens for a Safe Highway 9, were instrumental in creating an award-winning partnership and advancing this project from concept to concrete.
Following construction, a five-year research study was initiated. The SH 9 study was unique from many other studies evaluating the effectiveness of ungulate-highway mitigation in that the structures were located in mule deer winter range rather than along a migration path. In addition, crossing structures of the same type all had the same dimensions. This study evaluated success rates for each species at the crossing structures and conducted a before-after-control-impact analysis to determine how well the mitigation contributed to an observed decrease in WVC.
The research demonstrated that the mitigation helped to reduce WVC by over 90%. In addition, over 112,000 successful passages by mule deer were recorded at the seven structures over the five-year study period, with an overall success rate of 96%. Results confirmed the value of both the wildlife overpass and underpass designs in maintaining connectivity for mule deer in winter range and for all age and gender classes of the population; and underscored the influence of mule deer distributions in winter range on crossing structure use, regardless of structure type.
The study also established the value of the wildlife crossing structures and other mitigation for a number of other species, including elk, pronghorn, moose, bighorn sheep, white-tailed deer, black bear, mountain lion, bobcat, coyote, and other meso and small mammals. While the total number of successful passages by other ungulate species was much lower than for mule deer, success rates were high across species. The research detected some species-specific preferences for underpasses versus overpasses and determined the primary influences on each species use of the crossing structures.
The SH 9 research was instrumental in demonstrating the success of the wildlife crossing structures and fencing mitigation for both wildlife and motorist safety. While a recent emphasis has been placed on migration paths in the western U.S., monitoring research on SH 9 confirmed the need to also protect movements within winter range. Overall, the findings of this study and resulting recommendations will help transportation and wildlife agencies continue to site and design wildlife crossing mitigation systems for maximum success.