Wildlife carcasses recorded by Montana Department of Transportation, Idaho Department of Fish & Game, and U.S. Fish & Wildlife Service were aggregated to the nearest mile marker for major roads of the U.S. Northern Rockies. WGA connectivity flowlines were intersected with the road network and attributed to the nearest mile marker, along with their connectivity ranking, which indicates their expected relative importance to maintaining westwide connectivity. Values for potential risk factors, including average annual daily traffic (AADT), functional class, number of lanes, road surface width, landscape condition of surrounding habitat, ruggedness of surrounding landscape, and topographic position relative to surrounding landscape, were also attributed to mile markers. Landscape condition and topographic variables were calculated based on values observed within a half-mile radius of the focal mile marker. Landscape condition, an index of the degree of human modification of the landscape, is hypothesized to affect the likelihood of animals moving through adjacent habitat and attempting to cross the road at a particular site. Ruggedness (calculated as the standard deviation of slope values within a focal area) and topographic position (defined as the elevation of the focal point minus the mean elevation within a surrounding focal area, resulting in high values attributed to peaks and low values attributed to canyon bottoms) are expected to affect driver visibility from a particular site.Note that wildlife carcass collection and reporting protocols and frequency differ between states and among maintenance sections within states. Relative carcass counts should therefore be used only to explore general patterns and not for statistical inference.
