Olfactory repellents decrease the number of ungulate-vehicle collisions on roads: Results of a two-year carcass study

An experimental study was conducted to test the effectiveness of olfactory repellents (ORE) as a mitigation measure to reduce ungulate-vehicle collisions (UVC). In the first phase, an extensive field survey was undertaken while employing the Before-After Control-Impact (BACI) study design. On the basis of ungulate mortality, 134 road sections were monitored on foot along both roadsides once a week. The monitoring lasted fourteen weeks per year in both 2021 (Before period) and 2022 (After period). In the after period, 2022, ORE were applied within the impact segments. The second phase consisted of data verification and statistical analysis. The data revealed a decrease in UVC of 68%. The confidence interval of this estimate suggested, however, a great deal of uncertainty about the true value. Therefore, the data were pooled, and the Bayesian inference was applied. On the level of moderate evidence, ORE decreased the number of UVC by at least 43% and at most 60%. We also observed that the ORE effect was more pronounced in the first seven weeks after installation than in the following seven weeks, suggesting ungulate habituation to ORE. We have therefore concluded that for a short period (ideally corresponding to UVC peaks) ORE could be considered an effective safety measure for secondary roads.