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. 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.

In 2005, Massachusetts installed four culverts under a stretch of Route 2 in a wooded area between Boston and Cape Cod that enable raccoons, weasels and small deer to avoid the busy highway. In California, desert tortoises crawl through culverts specially constructed in the past few years along washes in the Mojave Desert.

At some crossings, fencing is erected to keep the animals off the highway and guide them toward the underpasses and overpasses. The underpass in Ohio will be installed over a natural gas pipeline that the butterfly follows along its migratory path.

There are about 400 wildlife crossings on land in the United States.

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.''

In Ohio, the Grizzled Skipper butterfly is an endangered species. There are about 25 of the black-and-white butterflies in the colony in southeast Ohio, and the forest is their only known habitat in the state.

Wildlife officials fear that the highway could snuff out the species if it blocks the butterfly from spreading to other habitats.

''We felt that this was going to isolate the population. That will doom it,'' said Lynda Andrews, wildlife biologist for Wayne National Forest.

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.

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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.

Wildlife crossings sometimes meet resistance.

Wyoming has decided against constructing overpasses for elk and other animals when it rebuilds and widens a highway through the Togwotee Pass 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.

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.''

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