Given are the years of the first and last official maps the routes appear on. These are not always the exact years the routes were created or eliminated.

The current numbering first appears in 1928. U.S. Route numbers were duplicated in those routes (1-63) but by the time numbers got to 72 (1934) they were being skipped, and not reused when eliminated (31, renumbered 112 in 1943 because it crossed US 31, and 29, absorbed by 17 in 1951). A major renumbering in 1955-1957 had three purposes:

• Remaining conflicts were eliminated (including 98, since US 98 entered Alabama after SR 98 was assigned in 1940)
• Most U.S. Routes got a single state number for their entire length (e.g. 15 along US 29)
• Directional parity was imposed, with odd numbers going north-south and even numbers going east-west (it seems that this was adopted as a policy for new routes by 1946)

It appears that numbering reached 210 by 1955-1957, but a few of these routes (204 206) were still proposed and don't appear until 1958 (and 1962: 200 201?).

Old numbers were generally reused as soon as possible, though this doesn't seem to be true for all numbers in the 1940s-1950s (e.g. 73). In particular, a lot of routes were eliminated in the years leading up to 1955-1957, leaving the following open for reuse in subsequent years:

• 1957-1958 through 151, also 157
• 1959 153 159 167
• 1962 175 199 201 203
• 1963 209-233
• 1964 235
• 1965 237-239
• 1967 227 (died 1964) 241-249
• 1969 251-253
• 1970 255
• 1972 257-269; 265 appears in 1973
• 1973 271?-275
• 1977 277
• 1979 277 (died 1977) 279
• 1981 281
• 1983 211 (died 1977)
• 1985 203 (died 1973 - was there another?) 275 (died 1979)
• 1987 283? 287; 285 appears in 1993
• 1989 37 (died 1983)
• 1991 271 (died 1989) 289? 291
• 1999 37 (died 1995)
• 2003 277 (died 1989) 295 297? 299; 293 appears in 2004

• 1957-1958 through 40, also 108 (replaced 107)
• 1959 44 58
• 1961 66 70 86; 62 appears in 1964
• 1962 88 92 100 102 110 112
• 1963 114 116 124 128 132
• 1964 136 138 140 142
• 1965 144 146
• 1967 148 152 154 156
• 1972 158 160 162 166
• 1973 170 176 186; 178 appears in 1975
• 1983 108 (died 1962 - there was probably another in between)
• 1985 140 (died 1977) 190
• 1987 192 194 (died 1981) 198
• 1989 138 (died 1987) 208 212?
• 1995 62 (died 1993)
• 1997 126 (died 1989) 138 (died 1993); 108 (died 1989) appears in 1999?

Partially missing numbers are: 62 108 (twice; location for the first suggested below) 186 190 192 198 (location suggested below) 213 285