Highway widening and underpass effects on vertebrate road mortality

What is it about?

Road widening (a.k.a. road dualling) and the presence of mitigation structures may have opposing effects on the number of animal-vehicle collisions. Their influence in tropical areas is poorly quantified, and we know little about how modifications of road structure affect fauna roadkill and mitigation. We evaluated how road widening and proximity to a wildlife underpass affect roadkill of medium and large mammals, using roadkill records from before and after the widening of 150 km of road with new and old wildlife underpasses. Roadkilled species were divided into three groups based on mobility and sensitivity to human disturbance. Four of 16 species exhibited significantly higher roadkill after widening. Roadkill near underpasses was generally higher than by chance, despite our expectation of reduction in roadkills. This result indicates that we must adopt more effective mitigation measures, such as appropriate fencing combined with underpasses.

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Why is it important?

Underpass versus roadkill: beyond use, toward efficacy Researchers and road agencies must work together to evaluate the effectiveness of mitigation strategies such as fences and underpasses (van der Grift et al. 2013). All interest groups (e.g., road ecologists, road concessionaires, road government agencies) should be involved in the process of designing monitoring programs for mitigation. This includes collecting information before and during road construction or widening, adequate replication, and appropriate spatial and temporal scales for evaluation (van der Grift et al. 2013). In the current study, the fences were not effective for most of the local medium- and large-sized mammals. Fences were too short, and species with climbing abilities easily overcame them (e.g., Puma concolor, Didelphis albiventris, and Procyon cancrivorus). Other species such as Mazama spp. can jump over 1.5 m. In this region, fencing would have to be at least 2 m high to effectively mitigate road mortality. After widening, roadkill of LMG and LMS groups was unaffected by proximity of underpasses. However, before widening, the average roadkill distance to an underpass was closer than would be expected by chance. This suggests that underpasses are near preferred crossing locations. Lack of fencing, however, does not prevent animal mortality in such locations.

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