Currently available devices that incorporate acoustic deterrents are often attached to delineator poles and triggered by the light of an approaching vehicle or the presence of large animals detected by thermal cameras, movement sensors, or other systems. The devices emit an artificial whistling sound in a frequency audible to the animals that aims to discourage wildlife, mainly deer, from crossing the road or railway. However, there is yet no convincing evidence for the efficacy of these devices in reducing accident risks.
Similarly, so-called deer whistles, which are mounted on the vehicle and produce sound via the airflow are intended to scare deer away so that no collision occurs. However, the sound may be ineffective because it is outside the audible range of the animal or is drowned out by traffic noise.
Combined acoustic and visual deterrents, also known as ‘virtual fences’, are also increasingly installed. Attempts are made to reduce the habituation effect by making it possible to vary the sound frequency or colour of the light (Figure 5.4.2). Studies show contradictory results on effectiveness (see Rationale box. Deterrents to wildlife).
Bio-acoustic approaches that, instead of a bell, chime or whistling sound, use a natural sound that animals associate with a real threat, such as human voices, dogs barking or a species-specific alarm show more promising results. Several studies show a strong flight response in deer and wild boar when human voices are emitted, but the effect may be context dependent. More studies, especially long-term monitoring, are needed to further develop and refine bio-acoustic systems to prevent animal-vehicle collisions.
Another method which has been proposed is a grooved or rippled road surface that produces a loud warning sound when vehicle wheels move over it. These sounds must be in the frequency range of target species for this to work. However, more detailed investigations are needed to verify its effectiveness and concerns about the impact of such a noise on the adjacent habitats have been raised.