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:
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.
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 Route 30.
(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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Faigin.
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