Frogs, salamanders, and other amphibians around the world face mounting threats from a devastating fungus, climate change, habitat loss—and road mortality. Among these, roads pose a uniquely immediate danger by cutting through critical migration corridors, allowing vehicles to crush millions of animals each year. 

Now, a new, first-of-its-kind study offers powerful evidence that a simple intervention—wildlife underpass tunnels—can dramatically reduce these amphibian deaths and help preserve ecosystems.

In research spanning more than a decade, scientists and citizens from the University of Vermont, the Vermont Agency of Natural Resources, and the local community assessed the effectiveness of two wildlife underpasses installed under a road in Monkton, Vermont. The results were striking: an 80.2% reduction in amphibian deaths.

“It was surprising. I knew that underpasses would work, but I didn’t think they would be that effective,” said lead author Matthew Marcelino, an ecologist at UVM. “And when we took climbing amphibians out of the picture—which in our context are primarily spring peeper frogs—we noticed a 94% decrease in mortality in the treatment areas."

The study was published online in the Journal for Nature Conservation and will be printed in the August edition of the journal.

Before & After Design

Using a rigorous “before-after-control-impact” (BACI) study design, the citizen scientists and researchers monitored amphibian road mortality over five years before the construction of the underpasses (2011–2015) and seven years after (2016–2022). They compared three zones: one with underpasses and wing walls (treatment); one covering the area at and beyond the end of wing walls away from the tunnels (buffer); and a control area far from the infrastructure changes.

The study team conducted standardized surveys during the amphibians’ brief spring migration windows, walking the road each rainy evening and recording every amphibian—alive or dead—across twelve species of frogs, toads, and salamanders. They found 5,273 amphibians including 1,702 spotted salamanders, nearly half of which were dead, and 2,545 spring peeper frogs, nearly 70% of which were dead. The death rates were much lower in the treatment areas—and the buffer areas too, showed that the animals were using the tunnels and not just being displaced to the ends of the walls.

This research provides the first long-term, peer-reviewed evidence that amphibian-specific wildlife underpasses in the northeastern U.S. are highly effective. It also highlights that the design details—like wall height and angles, tunnel layout, and material—really matter.

Roads Kill

Amphibians—frogs, toads, and salamanders—play vital roles in ecosystems and are highly sensitive to environmental disruption. In Vermont and much of the northeastern United States, many amphibians spend the majority of their lives hidden in upland forest soils or streams, fattening up and surviving long winters underground. But in early spring, they emerge en masse on warm, rainy nights to migrate from upland woods to ponds, wetlands, and flooded meadows to breed.

“It’s usually sometime between late March and late April,” explained study senior author and UVM professor Brittany Mosher. “Many species will breed in the same ponds. So it’s not just a single species migrating—it’s many, many species. And oftentimes, we see hundreds or thousands making this movement all at the same time.”

Unfortunately, roads are often built right between these forested uplands and aquatic habitats—exactly where amphibians must cross. “Planners—state and federal transportation planners—often build roads between these steeper forested upland habitats and nice flat aquatic habitats,” said Mosher, an ecologist in UVM's Rubenstein School of Environment and Natural Resources. “So the roads are placed exactly in the wrong spot if you were an amphibian planner.”

Because amphibians are small and slow-moving, they are especially vulnerable to vehicle collisions. Unlike larger mammals that might dart across a road in seconds, frogs and salamanders can take several minutes to cross—and with hundreds migrating at once, mortality rates can be staggering.

In Monkton, Vermont, local residents witnessed this firsthand. In 2006, community members from the Monkton Conservation Commission and the Lewis Creek Association went out to observe a known migration corridor. What they saw shocked them.

“They counted over a thousand dead animals on the road in just two nights,” Mosher said.

Takes a Village

That community concern helped launch a collaboration between residents, conservation groups, UVM scientists, and state agencies—led by long-time Monkton resident, State of Vermont wildlife biologist, and study co-author Steve Parren. The collaboration eventually led to the construction of two amphibian underpasses beneath a 1.3-kilometer stretch of road in Monkton. These structures, designed to allow amphibians to pass safely under the road during migration, were installed in 2015 with the support of the Vermont Fish and Wildlife Department and the Town of Monkton. The project cost $342,397—far lower than large mammal overpasses and underpasses, which can range from $500,000 to nearly $100 million per crossing. 

The Vermont design features two 4-foot-wide concrete tunnels with wing walls to guide amphibians to the tunnels and safely under the road. Using wildlife cameras, the Lewis Creek Association counted 2,208 amphibians using one of the underpasses in the spring of 2016. (It turned out that other animals were happy to use the tunnels too: the cameras captured bears, bobcats, porcupines, raccoons, snakes, and birds using the underpasses—suggesting they broadly benefit ecosystem connectivity.)

While the reduction in mortality for climbing amphibians like spring peepers was not statistically significant, it did decrease by 73%. Non-arboreal amphibians—those that travel along the ground—benefited the most from the underpasses, with consistent use and drastically fewer fatalities.

The research team emphasizes that the findings should serve as a model for road planners and policymakers across the country. “This study provides strong evidence that wildlife underpasses work,” Marcelino said. “We hope this will encourage transportation departments to include them in future plans, when building or repairing roads.”

Mosher added that these structures are not just helpful for amphibians—they are a signal that communities can come together to protect their local wildlife. “This story began with local community members who were engaged and concerned,” she said. “And it provides a view for how other communities can protect their amphibian populations too.”

The study highlights the critical role of long-term research, community engagement, and targeted infrastructure investment in supporting biodiversity. “Conservation takes commitment,” Marcelino added. “But when we invest in good tools and take the time to do it right, the payoff for ecosystems and wildlife can be enormous. These are beautiful creatures—so beautiful, so ancient.”