Montana ? Snowmobiles would become a permanent winter fixture in Yellowstone National Park under a proposal, drafted by park administrators and released Tuesday, that would allow 720 lower-polluting machines daily.

The policy change would continue rules in place for the past three winters, although actual snowmobile use has only averaged about 250 machines per day.

The National Park Service’s proposal is a defeat for conservation groups and some former park employees who had sought an outright ban on snowmobiles. The plan also would allow 78 buslike snowcoaches per day in the park; there is no current limit.

The park service proposed such a ban in 2000, but it was never enacted because of legal challenges. In the late 1990s, as many as 1,400 snowmobile users a day visited Yellowstone, contributing noise and air pollution that critics said was inappropriate for the country’s first national park.