PROVO — They have spread like a spiderweb across the land, a tangle of tire tracks that mar Utah's foothills, forests and canyons.

They are the marks of off-road vehicles.

In the past 30 years, off-road vehicle use in national parks has soared — from five million users in 1972 to 36 million in 2000. In some states, where there have never been limits on cross-country riding, the scars are deep and long. Vegetation has been uprooted, wildlife scared off.

Last week, the National Forest Service announced a proposal to restrict off-road vehicles to designated trails. The announcement was applauded by environmentalists and criticized by off-road enthusiasts who say it is becoming increasingly difficult to find a place to ride.

"It's always some demon out there tearing things up, that's the perception," said Richard Beardall, a member of Utah's Shared Access Alliance (USA-ALL), an off-roader's advocacy group. "People fail to realize that by closing down trails that have historically been open, they're affecting the quality of people's lives."

In Utah's Uinta National Forest, off-road vehicle use has been restricted to designated trails for the past 10 years. Those trails are often ignored however, and enforcement is difficult. The state's rapid growth has also posed a problem: subdivisions are creeping farther and farther up the hills. Some homes literally push up against the forest.

"People come home from school or work, and they ride in the hills behind their homes, not realizing they are riding on public lands," said Loyal Clark, a Forest Service spokeswoman.

The foothills above Lindon were especially popular among off-roaders a few years ago and criss-crossing trails began to overtake the hill.

"Instead of staying on a trail, anybody who wanted to make a trail was doing so, and it was really carving up the hillside," Lindon Mayor Larry Ellertson said.

Lindon's City Council passed an ordinance that prohibited off-roading in the foothills and later facilitated a sale of 200 acres of private land to the Forest Service. Ellertson said enforcement of the ordinance has been effective.

"I used to see lights up on the mountain, and many of them at all hours of the night," he said. "I don't see that as much anymore."

Those who ride dirt bikes and ATVs say they get a bad rap. Most say they stay on designated trails.

"They portray us like a bunch of crazy guys on ATVs that just came out of 'Mad Max,' " said Beardall. "I sure don't fit that criteria. I'm a down-to-earth, normal guy in a wheelchair. I have to have help to get on and assistance while I ride."