"Some of the illegal use is very infrequent. Some of it is crossing boundaries that may not be well marked," says Jerry Case, the service's regulations program manager. "We're certainly working on it. [But] there are probably just a handful [of parks] that have significant illegal use causing significant environmental damage."
The Cape Hatteras seashore is one facility that until recently had not conducted the required legal reviews to allow off-road vehicles, though it is doing so now.
"We certainly recognize that some [facilities] need to get back in line with the executive order and regulations, but for the most part I think we're doing all right," Mr. Case says.
Other branches of the Department of Interior, however, say illegal motorized vehicles are a serious national problem. The chief of the National Forest Service, for instance, has called it one of his top four management problems.
An Interior Department spokesman told Congress in July that 43 of the nation's federal "park units" allow snowmobiles, 25 allow off-road vehicles, and nine allow personal watercraft.
But he made no mention of any problem with the illegal vehicles highlighted by the internal survey.
Critics say the problem is acute and growing, even if not yet felt uniformly. "The pressure to increase these kinds of uses in national parks is constantly increasing," Mr. Barger says. "Our park lands are becoming more and more like isolated islands, with the demands for their use from everybody."
At least 113 of the nation's 388 park units report illegal off-road vehicles, the internal survey showed, and 85 of those report that the vehicles have caused environmental or other damage. Striking as those results are, they may understate the scope of the problem, says Sean Smith, public lands director for Bluewater Network. Only 273 of the 388 park units had responded to the survey. Parks like Yellowstone, Acadia, and Shenandoah National Parks had not responded.