How Effective Is Road Mitigation at Reducing Road-Kill? A Meta-Analysis

Rytwinski, Trina; Soanes, Kylie; Jaeger, Jochen A.G.; Fahrig, Lenore; Findlay, C. Scott; Houlahan, Jeff E.; van der Ree, Rodney; van der Grift, E.A. · 2016 · OpenAlex-citations

DOI: 10.1371/journal.pone.0166941

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Summary

This meta-analysis addresses the critical lack of reliable data regarding the effectiveness of road mitigation measures in reducing wildlife mortality (road-kill). With hundreds of millions of animals killed annually by traffic, transportation agencies face difficult decisions due to the wide variability in mitigation costs and the scarcity of evidence comparing the efficacy of over forty available measures. The study aims to quantify the relative effectiveness of different mitigation strategies, determine if effectiveness varies by taxon, and assess how study design influences reported outcomes. The authors conducted a comprehensive literature search across multiple databases, including ProQuest, Web of Science, and Google Scholar, identifying 50 primary empirical studies that quantified the relationship between road-kill and specific mitigation measures. The analysis included studies employing Control-Impact (CI), Before-After (BA), or Before-After-Control-Impact (BACI) designs. Data were extracted to calculate standardized mean differences (Hedges’ d) as the effect size, with adjustments made for sampling effort and construction phases. The final analysis utilized a complete dataset of 99 effect sizes, treating each estimate as independent, to evaluate predictors such as mitigation category, taxon, fencing type, and study design duration. The results indicate that road mitigation measures reduce road-kill by an average of 40% compared to controls. Fencing, whether used alone or in combination with crossing structures, was the most effective strategy, reducing road-kill by 54%. Conversely, crossing structures installed without fencing showed no detectable effect on reducing mortality. Effectiveness varied significantly by cost and target species; expensive measures were substantially more effective for large mammals. Specifically, the combination of fencing and crossing structures reduced large mammal road-kill by 83%, compared to a 57% reduction for animal detection systems and only a 1% reduction for inexpensive wildlife reflectors. The study concludes that inexpensive measures like reflectors should not be deployed until their effectiveness is validated through high-quality experimental approaches. The authors highlight significant gaps in current research, noting insufficient data to evaluate less common measures or the specific attributes of crossing structures. To improve future evaluations, the paper recommends rigorous study designs that include pre-mitigation data collection. Specifically, the authors advocate for a minimum study duration of four years for Before-After designs and either four years or four sites for Before-After-Control-Impact designs to ensure statistically defensible conclusions for road planners.

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• regulatory evaluation

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• Applied Guidance: countermeasure evaluation