The Bush administration yesterday gave local managers the authority to decide where visitors can use off-road vehicles in national forests, a move that could reshape how Americans experience the country's 155 forests and 20 grasslands.

The new rules -- which take effect in 30 days -- do not restrict or expand access for all-terrain vehicles, which have become increasingly popular, and they do not affect snowmobiles. But Forest Service Chief Dale N. Bosworth, who has identified "unmanaged recreation" as one of the four biggest threats to national forests, said the proposal will both satisfy visitors and protect public land.

"Our problem is, recreation has to be carefully managed," Bosworth said in a telephone briefing yesterday. "The rule itself doesn't open or close a single route. That's a local decision."

Rather than specifying criteria for designating routes for motorized vehicles, the rules instruct local officials to base decisions on public input, with the aim of minimizing environmental damage and conflicts with other users. This approach drew praise from riders of motorcycles, four-wheel-drives and other off-road vehicles -- whose numbers have risen from 5 million in 1972 to 51 million in 2001 -- but drew criticism from environmentalists.

Clark L. Collins, executive director of the Idaho-based Blue Ribbon Coalition, which represents 1,100 off-road recreation groups, said the plan "gives our folks some leverage" with forest managers who have balked at opening up land to motorized vehicles.

"It recognizes the popularity of off-highway vehicle recreation and just says it needs to be managed," Collins said. He added that a Forest Service official had assured him the new rules will allow off-road riders to lobby for expanded access in some areas.

The Forest Service allows motorized vehicles on 200,000 miles of forest roads and more than 36,000 miles of trails, but there is no uniform standard. Bosworth estimated the new designations, which should be completed in four years, would affect about half of all national forests.

Jason Kiely, executive director of the Natural Trails and Waters Coalition, said the Forest Service took "a baby step" toward tackling the problem of illegal off-road recreation on public land: "They pushed the hard decisions down to the local level and didn't give the local managers the clear direction they want and need."

In Georgia's Chattahoochee-Oconee National Forests, for instance, there are 133.5 miles of legal motorized trails, but forest officials estimate that users have created more than 550 miles of illegal trails. Taxpayers have spent nearly $1 million closing these routes and restoring land damaged by off-road vehicles.

Jack Troyer, a regional forester in the West, said he hopes off-road riders will enforce the new designations "by peer pressure," although the government can impose a maximum fine of $5,000 for illegal motorized recreation.