Missouri supplemental route

Prior to 1907, all road improvement activities in Missouri were undertaken by the individual counties, with little expertise or coordination between them. Amid growing automobile presence and insufficient road networks in Missouri in the ensuing years, the state legislature created a state highway department and the state highway commission as well as enacted various laws aimed at improving transportation in the state.[1] In 1920, recognizing that economic prosperity and growth are highly related to good transportation, the legislature passed a $60 million bond issue for road work.[3] This effort, along with the Centennial Road Law passed by the legislature in 1921, shifted highway building efforts in Missouri from the local level to the state level. As a result, the state highway commission undertook an aggressive road building campaign throughout the 1920s and 1930s aimed to get Missouri "out of the mud".[1]

In 1952, the state highway department embarked on its Missouri 10-Year Highway Modernization and Expansion Program. Through this effort, the state assumed maintenance responsibility of over an additional 12,000 miles (19,000 km) of secondary and farm-to-market roads previously managed by the counties.[1] The goal of the secondary highway system was to place state-maintained roads within 2 miles (3.2 km) of more than 95% of all rural farm houses, schools, churches, cemeteries and stores.[4] Most of Missouri's lettered supplemental highways are the result of this program.

Top: Missouri Supplemental Routes B and FF concur in Nodaway County. Bottom: Spur Route N detaches from its parent route in Cedar County.

The more major supplemental routes of the system are assigned 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 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 exceptions to this are 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.

Supplemental routes rarely run for more than a few miles, although they may cross county lines. A route's designation changes at a U.S. Route or Interstate highway; 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. However, when Route MM in Greene County crosses Route 360, it remains Route MM, only changing to 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. Designations are also reused, but not near one another. For example, Route D exists in the counties of Cole, Greene, Newton, St. Louis, and several others. Rarely is a designation reused in a county; exceptions include two highways designated Route Z in Randolph County and two highways designated Route K in Carter County.

In some cases, supplemental routes will share a concurrency with a state route or another supplemental route. For example, Route J in Howard County overlaps with Route 87 for about 0.25 miles (0.40 km) near Boonesboro, Route D in Greene County overlaps with Route 125 for a few miles, Route N and Route O in Franklin County overlap for 2.02 miles (3.25 km) between Catawissa and Robertsville, and Route CC in Moniteau County overlaps with Route C for a few miles.

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, Spur Route V in Butler County, and Spur Route Y in Montgomery County. Then, one connector route: Connector Route M in Jefferson 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, or from the use of letters representing county trunk routes in Wisconsin.

• Missouri Route M (Jefferson County)
• Special route