Walk this way

CONCORD -- The stretch of Route 2 next to the Concord landfill is among the busiest roads in suburban Boston: four lanes of fast-moving traffic, typically gathering speed as it heads to or from the sharp bend in the road at Crosby's Corner.

Before long, a structure could appear above the highway that will cause drivers to slow down for a better look: A tree- and shrub-covered overpass 25 feet wide and packed with earth on its sides, serving as a passageway for both wildlife and humans.

A formal proposal for the extraordinary bridge, which would link trail networks near Walden Pond, is scheduled to be issued next month by planners and designers from the University of Massachusetts at Amherst and the Metropolitan Area Planning Council.

In addition to hikers, a broad range of wildlife -- including flying squirrels, tree frogs, deer, and coyotes -- is expected to use the passageway. Black bears also have been known to visit the expansive wooded area near Walden Pond and are potential users.

Bridge planners stress that humans and wildlife probably would not use it at the same time, because animals that inhabit the suburbs are wary of people. A s a practical matter, they say, humans would not be using the bridge at night, when most of the animals are likely to use it.

"We believe that this has the capacity to be a very beautiful and iconic structure," said Steven Winter, senior project director for the Metropolitan Area Planning Council, the regional agency overseeing the project.

Matt Burne, land conservation coordinator for the Walden Woods Project, said the overpass would reconnect Henry David Thoreau's Walden Woods, which is sliced in two by the highway. Motorists will find the bridge a remarkable sight, he said.

"Because of the cultural and environmental significance of this area, we feel this ought to be something to behold," Burne said.

While wildlife tunnels under highways are increasingly common -- there are four already on Route 2 in Concord -- elaborate overhead passages, sometimes called land bridges, are rare in North America. Florida has one, over Interstate 75 in Ocala in the north central part of the state, and there are two over the Trans-Canada Highway in Banff National Park in Alberta.

Some larger animals, as well as those that dwell in trees, are wary of tunnels and are more likely to cross an overhead structure.

The 2,680-acre Walden Woods is where Thoreau came to study nature, garden, and write, from 1845 to 1847. He chronicled his experiences and philosophy in "Walden," one of the best-known nonfiction works in American literature.

"Some of us like to think this would be an appropriate, 21st-century innovation in the spirit of Thoreau -- connecting people to the environment," said Jack Ahern, head of the Department of Landscape Architecture and Regional Planning at UMass-Amherst and leader of the overpass design team.

A Federal Highway Administration grant of just under $200,000 paid for the bridge planning. Officials said they would seek federal highway money for construction.

A formal cost estimate has not been done, but Ahern speculated that the overpass would cost several million dollars.

Also undetermined is who would maintain the structure. Kathi Anderson, executive director of the Walden Woods Project, suggested corporate support might be solicited in a manner similar to the adopt-a-highway maintenance programs.

The state Department of Conservation and Recreation, which manages Walden Pond State Reservation, has voiced concern that the overpass could allow unauthorized access to the historic recreation area. Because of erosion on the banks of Walden Pond and other stresses on the environment, the department tries to limit access to the reservation to 1,000 people at a time. When the limit is exceeded, cars are not allowed in the parking lot, and pedestrians are turned away at the intersection of routes 2 and 126.

The Walden Woods Project, an educational and conservation organization founded by musician Don Henley, came up with the idea for an overpass, according to Anderson. She said the organization has been looking for a way to unite the Walden Woods. Route 2 was built through the area in 1935.

"We felt very strongly that Route 2 drives a wedge between the ecological unit that is the Walden Woods," Anderson said. "This is the cradle of the American conservation movement. What better place to do an innovative transportation project that addresses the needs of a sensitive environmental area?"

On one side of the bridge would be the closed Concord landfill, which is now a green space, next to Walden Pond. On the other side is Brister's Hill, an interpretive nature center, which is linked by trail to the Concord Town Forest. Through a series of connected trails, hikers also could reach Concord Center and the Great Meadows National Wildlife Refuge. In addition, the Bay Circuit Trail, a planned 200-mile path from Newburyport to Kingston, runs past the site of the planned overpass.

Concord Selectwoman Virginia McIntyre said the project would expose many more people to the area's special history and environment.

"It's an unusual project, but we do a lot of unusual things in Concord. If not here, where else?"

Robert Preer can be reached at preer@globe.com.