The tunnel, known as the Henry Street site, was constructed to help spotted salamanders (Ambystoma maculatum). More than three quarters of the local salamander population used it to cross the road, according to monitoring results.
Based on his experience with the Henry Street site and knowledge of other amphibian tunnels in Europe, Jackson has prepared and posted to a Web site his recommendations for appropriate tunnel design.
He says tunnels should be a two-foot-by-two-foot (0.6-meter-by-0.6-meter) box, open at the top, and fitted with an iron grate to allow sufficient air, light, and moisture into the tunnel.
In addition, barricades, such as fences or walls, should extend out from either side of a tunnel for about 100 feet (30 meters). This serves to guide amphibians toward the tunnels.
Jackson estimates that there are about a hundred amphibian tunnels in the U.S. today. But he said he is not sure how effective they are at protecting amphibians.
"There have been a lot built but not necessarily with careful design and careful planning, and no one knows how well they are working," he said.
The biologist said developers of housing subdivisions and industrial parks often approach him for amphibian-tunnel advice but then downgrade the designs in order to save money.
For example, Jackson said that many developers will not use iron grates or slots to allow moisture into amphibian tunnels, which the animals need to keep moving.
"So, very often it's considered a success just to get these installed. I haven't heard much back whether or not they are working," he said.
Biologists know the time of year and the conditions that cue to amphibians to breed. This means temporary road closures would be an effective way to prevent roadkill, according to Serrao, the Pennsylvania naturalist.
Serrao was instrumental in one such temporary road closure at the Delaware Water Gap National Recreation Area in the Pocono Mountains. It was there in March 2002 that he saw more than 700 amphibians squashed on just a 200-yard (183-meter) stretch of road.
"I talked to the park superintendent, and from now on they close off that road on the [first] four or five rainy nights of the spring, when the amphibians come out," he said.
According to the National Park Service, a biologist who monitored the road during the first closure in March 2003 counted nearly 500 amphibians successfully crossing the road.
Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES