BLM trails are divided into four classes:

• Type I: Very easy, paved, all-weather. Family car OK.
• Type II: Not paved, but usually improved. May be rough or rutted. High-clearance 4WD vehicles desirable.
• Type III: Unimproved dirt, often with rocks, steep hills, and mud. 4WD required.
• Type IV: Trails for mountain bikes, dirt bikes, and ATVs. 2WD or 4WD rigs not authorized.

Source: 4-Wheel-Freedom_, by Brad DeLong, published by Paladin Press.