From Massachusetts to Montana, states are adding highway overpasses, underpasses and fences for bears, moose, panthers, deer, frogs and butterflies, to cut crash numbers and ease possible threats to the existence of species.

"The amount of traffic is increasing; the density of roads is increasing. Roads are getting into habitats and rural areas where 10 years ago there was hardly anybody," said Tony Clevenger, a research scientist for the Western Transportation Institute at Montana State University. "It's one of the biggest conservation challenges in the 21st century."

In Montana, 41 underpasses are being built on a 56-mile stretch of U.S. Highway 93 so that bears, cougars, wolves, moose, elk, deer and wolverines can cross. There will be a landscaped 150-foot-wide overpass for grizzlies. Two were killed by vehicles in that section of road in the past four years.

Ohio will install a $1.7 million underpass on a new highway that will cut through the Wayne National Forest, to protect a tiny colony of butterflies and give safe passage to black bears, bobcats and deer. Construction of the highway will begin as early as 2007.

Wyoming, meanwhile, has decided against constructing overpasses for elk and other animals when it rebuilds and widens state highway 287-26 through Togwotee Pass between Dubois and Jackson because of the expense and because the required fencing would mar scenic views. Instead, the state will enlarge some culverts and reduce the speed limit from 65 miles an hour to 55 mph.

"We're stewards of the taxpayer dollars," said Cody Beers, public involvement specialist for the Wyoming Department of Transportation.

At some crossings across the country, including Wyoming, fencing is erected to keep the animals off the highway and guide them toward the underpasses and overpasses. Wyoming also uses special sensors linked to signs on some stretches of highway to alert motorists to the presence of wildlife.

Across the country, there are about 400 crossings designed specifically for wildlife.

Without crossings, traffic, noise, pollution, vibration and heat can block migratory paths and separate animal populations. That can lead to more inbreeding, loss of genetic diversity and possible extinction.

"Underpasses and overpasses are not just to cut down road kill, but to maintain the natural populations of these animals," said Richard Forman, professor of ecology at Harvard University. "One of the invisible things about highways is the fragmentation effect. We divide the land up into little squares and little polygons. It's like creating a megazoo."

Florida, which has more than 80 such crossings, plans at least 10 more within the next three years. A series of underpasses and culverts in the Everglades is credited with helping fuel the rebound of the Florida panther. There are between 70 and 100 panthers compared with 30 two decades ago.

In 2004, a crossing was built near Tallahassee for the striped newt, a small salamander. And near Boca Raton, lights were embedded in the center line of the beach highway and the streetlights turned off during hatching season to keep baby turtles from being lured to the road and run over.

Patty Cramer, a research associate at Utah State University, has surveyed biologists, engineers, wildlife experts and transportation officials to try to determine the effectiveness of wildlife crossings. Cramer said animals were using 68 of the 70 crossings in the survey.

Clevenger's study of Canada's Banff National Park indicated that wildlife crossings and fences reduced road kills by 80 percent.

And there is some indication crossings may be making roads safer for motorists.

In 2004 there were 300,000 animal-vehicle accidents, with 195 motorists killed. That's down from 316,000 accidents and 201 deaths in 2003. Property damage occurred in 282,000 of the 2004 accidents, down from 305,000.

Joseph Menicucci Jr. wasn't hurt when his car hit a moose in 2002 on Interstate 15 in Idaho, but he had nightmares about the crash for several weeks.

The moose rolled off the bumper of his Chevrolet Lumina, onto the hood and into the windshield. The shattered glass tore the animal to pieces. Then a Volvo slammed into the moose carcass, went airborne and landed in a shower of sparks.

"When I looked down, I was covered in blood. I thought it was my own," said Menicucci, a chemical engineering student at Montana State University.

Trish White, a director for the Washington-based Defenders of Wildlife, said states need to build more crossings. But more importantly, she said, states need to locate new roads in places that minimize the impact on wildlife.

States have gotten a nudge from the federal government. The transportation bill enacted into law in June encourages them to take natural resources into account when planning transportation projects.

"It's dangerous for us to think that crossings are the panacea, the silver bullet that is going to solve all the problems," White said. "They're big Band-Aids."