Wildlife crossings – overpasses, underpasses, and other structures that span linear infrastructure – are known to reduce wildlife-vehicle collisions, mitigate climate change, and promote connectivity between wildlife populations.
In the U.S., record funding from the Infrastructure Investment & Jobs Act of 2021 has led to a big increase in the number of wildlife crossing projects being proposed and implemented. With this increase in crossing structures comes an obvious question: how well do wildlife crossings work?
Marcelino, M.R., Parren, S.G. and Mosher, B.A. 2025. Assessing the efficacy of wildlife underpasses in mitigating amphibian road mortality: A case study from the northeastern United States. Journal for Nature Conservation 86:126901.
While studies on the effectiveness of wildlife crossings often focus on large-scale overpasses for large mammals, smaller underpasses also have the potential to keep populations connected. They can support smaller species, and usually require less time and money to install than larger overpasses. This makes them ideal tools for mitigating habitat fragmentation – if they’re shown to work well.
A new study looks at the effectiveness of small underpasses for amphibians, using a 1km stretch of road in Vermont that bisects critical wetland and upland habitats. In 2015, two underpasses were installed, and for the next seven years amphibian surveys were conducted to determine how many and what kind of amphibians were using the underpasses. These surveys were then compared to surveys conducted in the five years prior to installation, in a BACI (Before-After Control-Impact) experimental design that is a key element for determining efficacy of wildlife crossings.
Over the course of the study, over 5,000 amphibians were encountered crossing the road. When comparing mortality before and after underpass construction, there was an 80% decrease in mortality overall, with a 94% decrease for non-arboreal species and a 74% decrease for arboreal species. A total of 12 amphibian species were encountered during surveys, including blue spotted salamanders and four-toed salamanders, which are classified as uncommon and rare, respectively, in the region.
The design of the underpasses also provided insight. The lengths and angles of the walls funneling animals toward the underpasses created a buffer zone along the road between underpasses. While there was no increase in road mortality in the buffer zone, neither were any benefits of the treatment area seen there. This suggests that individuals would benefit from being kept out of the buffer zone by walls that are angled as far away from the road and as long as possible.
Amphibians are increasingly threatened by habitat loss and fragmentation, and are especially vulnerable given that they often depend on both upland and lowland habitats. Underpasses represent a highly effective management tool for keeping them off the roads that may bisect critical habitat. With proper consideration for design, size, and location, they can be a useful and relatively simple way to maintain population connectivity.
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