The negative impact of roads and railways on nature and landscape is a well-known fact. The construction of linear infrastructure leads to the fragmentation of previously functional parts of the landscape, direct loss and encroachment of habitats and disruption of the water regime in the surrounding area. It also causes loss of balance in the ecological functions of the landscape, including interruption of its movement permeability for animals. Among the most visible negative effects are animal-vehicle collisions, which can affect the viability of local populations of both common and endangered species. We tested the effectiveness of two types of animal deterrents that combine optical-acoustic and optical-olfactory stimuli and were designed to decrease the number of roadkills. We performed a Before-After continuous cadaver monitoring experiment (for 21 months before treatment and 18 months after treatment) on seven test sections of roads (with total length of 18.6 km), supplemented by Control-Impact data from 88 km of control road sections. Our results indicate that the tested deterrents did not have a statistically significant effect in reducing mortality. These findings highlight the need for additional ethological investigations to determine whether animals respond to the deterrent stimuli and should be carried out prior to any field testing. Robust, evidence-based field evaluations of mitigation measures are then necessary to ensure that only devices proven to effectively reduce animal–vehicle collisions, save lives, and prevent associated ecological and economic losses are implemented along roads.