The forest routes are part of a vast system of roads that provide access to national forest lands. They're used in logging, mineral extraction, livestock grazing, etc. They provide access to campgrounds, hiking trails, and other recreational resources, as well as being about the only access to a good deal of private land. The particular numbering system varies quite a bit from national forest to national forest. All route numbers are shown on the Motor Vehicle Use Map, which also shows the level of maintenance of the route.
There is a general hierarchy of routes in a national forest. The first level is a "Forest Highway," which are are generally maintained to be accessible to all vehicles. Definitely gravelled and often paved, Forest Highways are often not signed as such when they are designated along a state or US route. These are generally routes that are of some significance regarding the outside world, rather than just within the forest.
According to the Code of Federal Regulations, Forest
Highways are designated by number and name. Numbering is
consecutive within each state, and no number is to be used more than once in
each state. Other road numbering systems (i.e. those used outside the forest)
are disregarded in numbering the Forest Highway routes.
(CFR Title 23 §660.205)
There is a second class of forest routes that are maintained to low-clearance standards for passenger cars (i.e. you might not be able to travel at highway speeds, but with a little care, you should be able to get most vehicles up the things. These normally do not have formal shields, but have numbers are often posted horizontally on signs.
There are basically five levels of maintenance of forest routes generally open to the public:
In the Angeles National Forest, the numbers have the form 3N02. These routes are often trunk routes for the forest highways.
The secondary roads often connect to still smaller roads, which are "maintained" for high-clearance vehicles only. These will generally lead from the trunk roads to timber sales, lakes, meadows, and random locations that just happen to have roads to them. There are a lot of these on the maps, though they are not often marked on the ground, although you may occasionally find a vertical string of numbers on a 3 inch wide fiberglass post.
Although not a topic of this website, there are sites that address Forest Road numbering. For example, http://tchester.org/sgm/lists/anf_map_roads.html explores the numbering of Forest Roads and Trails in the Angeles National Forest. This site notes that:
The numbering of primary Forest Roads, such as FH59 (Angeles Forest Highway), FH61 (Angeles Crest Highway), and FH62 (San Gabriel Canyon Road) is simple sequential numbering.
Secondary roads and trails appear to be numbered based on the township and range system, assigned by the federal government survey as part of "settling the West". The township and range numbers are given on the Forest Map and on every USGS 7.5' x 7.5' topo map. The "baseline" for this system in the Angeles National Forest is the one going through San Bernardino Peak. A new township begins every six miles north and south of this line. Ranges are variable size east and west of this line. On the Forest Service Map, the townships are identified by black letters like "T. 4 N." on the left and right sides (you can also see these numbers on Thomas Bros. Maps). The ranges are identified by letters like "R. 6 W." on the top and bottom. The roads are given numbers that are based on the township in which the northern end of the road is located. The trails are given numbers based on the range in which the trailhead is located.
Secondary Forest Service roads in the ANF are given the designations xNyy, where x is a number from 1 to 8 and yy is a two-digit number. The number x is assigned based on the township, followed by the N because the Angeles National Forest is north of San Bernardino Peak, and hence in the northern townships. The number yy is assigned generally from east to west, but not always. A few short spur roads are designated by appending the letters A, B, C, etc. to the main road's designation. Thus 7N23B is a short spur road originating at 7N23. Similar to the assignment of the number yy, the letter assignment does not always seem to follow a logical pattern.
A few roads and trails have decimal points followed by another digit. The significance of this is unknown. For example, 3N26.2 exists with no 3N26 on the map, and 3N09.2 and 3N09 are two different roads shown on the map. Curiously, the road sign for 3N17 at 3 Points says 3N17.1, and the map also gives the designation of 3N06.1 and 3N09.1 in inset maps for the roads designated 3N06 and 3N09 on the main map.
Forest Service trails in the ANF are given the designations xxWyy, where xx is a one- or two-digit number from 7 to 18 and yy is a two-digit number. The number x is assigned based on the range, followed by the W because the ANF is west of San Bernardino Peak, and hence in the western ranges. The number yy is assigned generally from north to south, but not always. The x portion of the name of the roads seems to always be based on the northern end of the road. However, the x portion of the name of the trails is not based on a similar cardinal direction. The x could be based on the trailhead.
The orientation of the sign is significant. Roads maintained for passenger vehicles have signs with horizontal numbers. Roads not maintained for passenger vehicles, i.e., low clearance and 2WD, but possibly passable by high clearance and 4WD have signs with vertical numbers.
Over on AARoads, Tom Fearer noted: Forest Route numbers generally follow a
convention based off the Township they originate in. All National Forests are
charted in a Township and Range grid, in the case of the former they span 6
mile sectors. For roads like Forest Route 10 they also carry a Forest Road
Number which denotes it's Township point of origin. Much of Forest Route 10 is
internally numbered as Forest Road 6S10 denoting it's point of origin is in
Township 6S of Sierra National Forest. Forest Service Roads are given numbers
based off where what Township the north terminus is located in. Spur routes of
a mainline Forest Route carry a letter designation at the end hence why Beasore
Loop is signed is numbered Forest Road 6S10X. Most National Forests tend to
place two digit route shields on Forest Routes which are suited to travel by
conventional vehicles which includes run of the mill cars. In the case of the
Sierra Vista Scenic Byway while there are rough roads in it's alignment all of
them can be traversed by a normal low clearance car.
(Source: Tom Fearer (Max Rockatansky) on AARoads, "Re: Sierra Vista Scenic Byway; Sierra National Forest Routes 10, 7 and 81", 8/7/2019)
As noted, the site http://tchester.org/sgm/lists/anf_map_roads.html gives much more information on this. This numbering system appears to be used in other national forests as well, although sometimes the posted route number is only a portion of the full route number, or is a different number entirely.
For example, some forest roads in the Sierra National Forest only post the
last two digits of the route number, making Forest Route 6S10 into Forest Route
10. A few others even post different numbers than the original number: For
example, Forest Route 13S09 in the Sequoia National Forest is posted as Forest
(Source: Tom Fearer on FB, 7/28/2019)
[Much of the information on Forest Highways was provided in an m.t.r posting by Sam Smith.]
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