What We Miss on the Road: Visual Attention and Species Detectability in Roadkill Surveys

Wildlife-vehicle collisions pose a substantial threat to biodiversity, and yet roadkill counts--one the main ways used to quantify mortality--are often susceptible to underestimate true collision rates due to imperfect detection. This study investigates how carcass size and survey methodology affect detection rates during roadkill surveys. Using taxidermied specimens placed along roads, we compare roadkill detection in standardized surveys vs. opportunistic conditions that mimic citizen science data collection, in which observers are not asked to focus on roadkill search alongside roads prior to the survey. Results show that detection probability increases with body mass but declines sharply when observers are not explicitly focused on locating roadkill, with informed participants up to 27 times more likely to detect carcasses than uninformed ones. Even large-bodied species, such as red foxes and European badgers, were frequently missed in opportunistic contexts. By applying species-specific detection probabilities and carcass persistence estimates to a regional citizen science roadkill dataset (Faune-AuRA), we reveal that reported carcass counts represent as little as 0.8-5% of estimated collision numbers. These results demonstrate that roadkill counts, regardless of survey methodology, may substantially underestimate road mortality and distort assessments of species-specific vulnerability in transportation ecology.