Bureau of land management definition
(BLM) The United States government agency within the U.S. Department of the Interior which has divided trails into four classes: Type I is at least 7 feet wide (enough for a family vehicle), paved, easy to traverse. Type II is also at least 7 feet wide, not paved, but is usually improved. However it may be rough or rutted and contain washboards . It is best travelled by high-clearance 4WD vehicles. Type III is a narrow unimproved dirt road, often with rocks, steep hills, and mud over which only 4WD should use. Type IV trails are for mountain bikes, dirt bikes, and ATVs. 2WD or 4WD vehicles are not allowed.