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A supplemental route is a state secondary road in the U.S. state of Missouri, designated with letters. Supplemental routes were various roads within the state which the Missouri Department of Transportation was given in 1952 to maintain in addition to the regular routes, though lettered routes had been in use from at least 1932. The goal of the secondary highway system was to place state-maintained roads within 2 miles (3 km) of more than 95% of all farm houses, schools, churches, cemeteries and stores. The four types of roads designated as Routes are:Farm to market roads
Roads to state parks
Former alignments of U.S. or state highways
Short routes connecting state highways from other states to routes in Missouri
Supplemental routes make up 19,064 miles (30,681 km) (59%) of the state highway system.
Missouri supplemental route Wikipedia
The more major supplemental routes of the system are ones assigned with single-letter designations (such as "K"). Minor branch routes and farm-to-market roads, which often end at county roads or are former alignments of the other highways, are typically assigned with two-letter designations consisting of two of the same letter (e.g. "KK"). Additionally, combinations of letters may be used, but always with A as the first letter (such as "AD"); the only exception of this is on Route BA in western St. Louis County. Combinations beginning with the letter R are also used for routes that connect with state parks or other recreational facilities, which is the only use of R on the system; Route AR, a non-recreational route south of Bakersfield, is the only exception to this.
The vast majority of the highways in the system are designated with 19 letters of the alphabet. The letters "G", "I", "L", "Q", and "S" are not used because of the potential confusion with other letters and numbers. The only current use of X is on Route AX in Macon County.
These routes rarely run for more than a few miles, although they may cross county lines. At a U.S. Route or Interstate highway, they change their letters; for example, in Greene County, southbound Route J crosses U.S. Route 60 and becomes Route NN, and Route M in Cole County becomes Route J after passing U.S. Route 50.. But when Route MM (Greene County) crosses Route 360, it remains Route MM, only changing into Route B when crossing Interstate 44. Route J in Boone County is one of the few exceptions to this, as it continues past U.S. 40 for three miles before becoming Route O upon passing I-70. The names are also reused, but not near one another. Rarely is a letter re-used in a county; there are two Route Z's in Randolph County and two route K's in Carter County. Route D exists in the counties of Cole, Greene, Newton, St. Louis, and several others.
Supplemental routes are signed by black letters on a white background with a black border. Rarely, the shields will be marked with banners such as EAST, WEST, or END. There are no business or bypass routes for the roads; however, seven examples exist of spur routes: Spur Route N in Cedar County, Spur Route C in Gentry County, Spur Route C (to Union Covered Bridge State Historic Site) in Monroe County, Spur Route K (into East Lynn) in Cass County, Spur Route AA (to the Sappington Cemetery State Historic Site) in Saline County, Spur Route J (to Sigsbee) in Shelby County, and Spur Route Y in Montgomery County.
It is erroneously believed that due to these roads being designated by letters rather than numbers and their existing in more than one county that these roads are county roads, not state highways, with some businesses and residences located on these roads saying their address is "County Road A" for example. This may have also arisen from the signage used prior to the early 1960s, where the letter was painted black against a white background, with the words "STATE ROAD" above the letter and the county name (in all capital letters) below the letter.