36 C.F.R. § 7.52
(a) Snowmobiles.
(2) On roads designated for snowmobile use, only that portion of the road or parking area intended for other motor vehicle use may be used by snowmobile. Such roadway is available for snowmobile use only when the designated road or parking area is closed by snow depth to all other motor vehicle use by the public. These routes will be marked by signs, snow poles, or other appropriate means.
The park Superintendent shall determine the opening and closing dates for use of designated snowmobile routes each year. Routes will be open to snowmobile travel when they are considered to be safe for travel but not necessarily free of safety hazards.
[49 FR 29375, July 20, 1984]