Motorized outdoor enthusiasts are converging in increasing numbers on Western public lands — not only in areas marked for such outdoor enthusiasm, but in wilderness areas where rules against off-roading are nearly impossible to enforce. Registration of all-terrain vehicles and motorbikes in four Western states tripled from 1998 to 2006. The surge is traceable to the booming outdoor-recreation industry, as well as the culture of sprawl: In some places, houses have been pushed out so far that federally owned land is just a big backyard — albeit a public backyard where no individual has to take the specific blame for vehicle-aggravated erosion and water pollution. Off-roaders deny criticism that they’re out to defile untouched nature, arguing that public land is there for public use. “[Groups lobbying for wilderness designations] think it has to be kept in this pristine state,” says one motorcycle-shop owner. “These people don’t even use it.” Which is, of course, the point.