This is the system used by Peter Massey and Jeanne Wilson in their 4WD Trails, 4WD Adventures, and Backcountry Adventures books.
|1||1||The trail is graded dirt but suitable a normal passanger vehicle. It usually has gentle grades, is fairly wide, and has very shallow water crossings (if any).|
|2||2||High-clearance vehicles are preferred but not necessary. These trails are dirt roads, but they may have rocks, grades, water crossings, or ruts that make clearance a concern in a normal passenger vehicle. The trails are fairly wide, making passing possible at almost any point along the trail. Mud is not a concern under normal weather conditions.|
|3||3||High-clearance 4WDs are preferred, but any high-clearance vehicle is acceptable. Expect a rough road surface; mud and sand are possible but will be easily passable. You may encounter rocks up to 6 inches in diameter, a loose road surface, and shelf roads, though these will be wide enough for passing or will have adequate pull-offs.|
|4||4||High-clearance 4WDs are recommended, though most stock SUVs are acceptable. expect a rough road surface with rocks larger than 6 inches, but there will be a reasonable driving line available. Patches of mud are possible but can be readily negotiated; sand may be deep and require lower tire pressures. There may be stream crossings up to 12 inches deep, substantial sections of single-lane shelf road, moderate grades, and sections of moderately loose road surface.|
|5||5||High-clearance 4WDs are required. These trails have either a rough, rutted surface, rocks up to 9 inches, mud and deep sand that may be impassable for inexperienced drivers, or stream crossings up to 18 inches deep. Certain sections may be steep enough to cause taction problems, and you may encounter very narrow shelf roads with steep drop-offs and tight clearance between rocks or trees.|
|6||6||These trails are for experienced four-wheel drivers only. They are potentially dangerous, with large rocks, ruts, or terraces that may need to be negotiated. They may also have stream crossings at least 18 inches deep, involve rapid currents, unstable stream bottoms, or difficult access; steep slopes, loose surfaces, and narrow clearances; or very narrow sections of shelf road with steep drop-offs and possibly challenging road surfaces.|
|7||7||Skilled, experienced four-wheel drivers only. These trails include very challenging sections with extremely steep grades, loose surfaces, large rocks, deep ruts, and/or tight clearances. Mud or sand may necessitate winching.|
|8+||8-10||Stock vehicles are likely to be damaged and may find the trail impassable. Highly skilled, experienced four-wheel drivers only.|
Note: Trail ratings are highly subjective. A trail that is easy for an experienced driver with a highly modified vehicle may be impassable for a novice driver in a stock vehicle. Weather conditions may also significantly affect the difficulty of a trail. Also, the difficulty of a trail may change over time, as trail use, weather, and maintenance modify the trail. Always check with the managing organization and/or recent users before planning a trip on a trail and scout ahead on foot if you are not sure of current conditions.