36 CFR § 261.13 - Motor vehicle use.
After National Forest System roads, National Forest System trails, and areas on National Forest System lands have been designated pursuant to 36 CFR 212.51 on an administrative unit or a Ranger District of the National Forest System, and these designations have been identified on a motor vehicle use map, it is prohibited to possess or operate a motor vehicle on National Forest System lands in that administrative unit or Ranger District other than in accordance with those designations, provided that the following vehicles and uses are exempted from this prohibition:
(c) Over-snow vehicles;
(d) Limited administrative use by the Forest Service;
(e) Use of any fire, military, emergency, or law enforcement vehicle for emergency purposes;
(f) Authorized use of any combat or combat support vehicle for national defense purposes;
(g) Law enforcement response to violations of law, including pursuit;
(h)Motor vehicle use that is specifically authorized under a written authorization issued under Federal law or regulations; and
(i) Use of a road or trail that is authorized by a legally documented right-of-way held by a State, county, or other local public road authority.