How did the toad hop across the road?

About a quarter of the way from Naples to Fort Lauderdale on this remote cross-Everglades highway in Florida, the interstate passes over a grassy culvert.

To most motorists zooming past at 70-plus miles an hour, the bridge is nothing more than a thud-thud under their radials.

But if they actually pulled over to the side the road and clambered down the steep embankment, they would find themselves face to face with an environmental innovation that is changing the way highways are being designed around the world.

Here in Florida, they call it a panther crossing. In Canada, the same idea works for elk and deer. In England, a much smaller version helps keep migrating toads in hopping form.

The basic strategy is to create a safe, natural means for wildlife to cross a road without endangering their own lives and those of unsuspecting motorists.

Highway planners and ecologists in Florida came up with the idea of a wildlife underpass more than a decade ago when major improvements to Alligator Alley (see map) threatened to isolate a large section of the Big Cypress National Preserve.

They needed to find a way to keep animals off the road. But they also needed to allow animals - like the endangered Florida panther - to continue to hunt and roam in their native habitat on both sides of the highway. The solution: fence the entire length of the highway and funnel the wildlife into culverts passing safely under the traffic.

It wasn't cheap. Thirty-six culverts were constructed along a 40-mile section of Alligator Alley at a cost of $13 million.

But today, the project is hailed as a shining success and has been studied by engineers and ecologists facing similar problems in Canada, Mexico, Australia, and countries throughout Europe. Some ecologists - including pioneering environmentalists in the Netherlands - have taken the concept one step further with the development of wildlife overpasses.

It is all aimed at helping to reduce the negative impact of highways on natural ecosystems.

The crossing at mile marker 78 on Alligator Alley is carpeted with grass and ferns and affords skittish animals a wide view all the way across to the other side. To the north, it leads into the cover of a dark cypress forest. To the south, it passes through a line of willows into a sawgrass wetland surrounded by slash pine and cypress trees. The only apparent drawback is that every three to five seconds a car or 18-wheel tractor-trailer roars overhead. It fills the crossing with a jolting whoosh and rumble.

Nonetheless, the crossings work. Fresh deer and raccoon tracks mark a muddy game trail that snakes through the culvert. A 10-foot fence topped by three strands of barbed wire prevented me from checking the entire crossing for panther tracks. But ecologists working with radio tracking collars and heat-sensing cameras have verified that panthers and other animals are using the culverts.

"All the animals that are down there are using the crossings," says Gary Evink, an ecologist with the Florida Department of Transportation. "We have everything from alligators, to wading birds, to bears and panthers."

Mr. Evink and his colleagues at the Florida Department of Transportation are leaders in a campaign to encourage sensitivity among highway engineers to the ecological effects of their work. Florida is sponsoring an international conference on transportation and wildlife set for this September in Missoula, Mont.

The idea of helping wildlife cope with disruptions caused by highways is still a new concept. Experts are finding that highways create significant barriers to the natural interconnection among animals within a large region.

Busy highways can split wildlife into small, fragmented populations that are much more vulnerable to population fluctuations because they've lost their ability to roam and interact.

"One percent of the United States is covered by roads and roadsides. That is an area about the size of South Carolina," says Richard Forman of Harvard University in Cambridge, Mass., an expert on the impact of roads on wildlife. "Society hasn't come to grips with the ecological effects of this road system, but it is beginning to."

Some experts suggest examining the problem from an animal's viewpoint. On a typical four-lane highway carrying 20,000 vehicles a day, a car or truck would pass every 4 seconds. "Not many animals are going to get across that road - not alive anyway," says Paul Garrett, an ecologist with the Federal Highway Administration. "Roads effectively can become very solid barriers to a lot of species of wildlife."

At Banff National Park in Alberta, Canada, ecologists are working to mitigate the impact of widening the Trans-Canada Highway to four lanes. They used a combination of fencing, wildlife underpasses, and overpasses, and recorded a 96 percent reduction in road kill of elk and deer.

Grizzly bears, wolves, and cougars regularly use the underpasses. Bruce Leeson, of Parks Canada, says he is confident that the park's two new $2 million overpasses will be just as popular. He says wolves and grizzly bears are very wary animals. "It takes them time to find these new structures," he says. "Then it takes them time to get up the courage to go through them."

In Florida, transportation officials are planning construction of their first wildlife overpass. It will be built south of Ocala and will extend over I-75 at a cost of about $2.6 million. One goal is to encourage bears to roam from the Ocala National Forest to Florida's western coast, where bear populations are isolated and in decline.

In another innovative project, Florida officials are beginning work on a means to keep frogs, toads, snakes, and alligators off State Road 441 south of Gainesville where the road bisects the Paynes Prairie State Wildlife Preserve.

The plan calls for construction of a three-foot-high wall with an 18-inch lip protruding outward to block the slippery critters from crawling or hopping out onto the road. Instead, they will be directed to four culverts under the highway.