AASHTO adopted the following in 1937: http://www.gbcnet.com/ushighways/history/new_signing_policy_on_us_routes.pdf
QuoteNo new U. S. route located wholly in one State shall be established. U. S. routes, less than three hundred miles in length, heretofore established and located wholly in one State, shall be eliminated either by consolidation with other U. S. routes or by reverting to State routes, as rapidly as the State Highway Department and the executive committee of the American Association of State Highway Officials can reach agreement with reference thereto.
How well has this been followed? Let's see:
(lengths and years from us-highways.com)
still exist:
*201: Maine, 157 mi, 1926-
*211: Virginia, 61 mi, 1926-
*117: North Carolina, 109 mi, 1935-
*130: New Jersey, 83 mi, 1926-
*341: Georgia, 226 mi, 1926-
*46: New Jersey, 75 mi, 1954- (entered Pennsylvania until 1954, so not really a violation)
*350: Colorado, 80 mi, 1926-
*57: Texas, 103 mi, 1970-
violation*158: North Carolina, 347 mi (thus never a target), 1940- (entered Virginia until 1940, so not really a violation)
*360: Virginia, 223 mi, 1933-
*264: North Carolina, 199 mi, 1932-
*266: Oklahoma, 42 mi, 1926-
*171: Louisiana, 179 mi, 1926-
*175: Texas, 114 mi, 1931-
*181: Texas, 140 mi, 1926-
*290: Texas, 276 mi, 1926- (not a target until it was truncated from 591 mi in 1992)
*92: Florida, 177 mi, 1926-
*192: Florida, 75 mi, 1926-
*96: Texas, 170 mi, 1939-
violationextended into other states:
*311: North Carolina, 78 mi, 1935-2013 extended into Virginia
*319: Florida, 80 mi, 1933-1937 extended into Georgia
*131: Michigan, 271 mi, 1926-1980 extended into Indiana
*441: Florida, 137 mi, 1926-1948 extended into Georgia
*49: Mississippi, 312 mi (thus not a target after 1927 extension, at least if 49E or 49W is included), 1926-1963 extended into Arkansas
*58: Virginia, 508 mi (thus never a target), 1931-1996 extended into Tennessee
*258: North Carolina, 146 mi, 1932-1940 extended into Virginia
*159: Kansas, 28 mi, 1935-1941 extended into Nebraska
*460: Virginia, 257 mi, 1933-1946 extended into West Virginia
*166: Kansas, 177 mi, 1926-1945 extended into Missouri
*74: North Carolina, 345 mi (thus not a target after 1938 extension), 1926-1987 extended into Tennessee
*377: Texas, 405 mi (thus not a target after 1951 extension), 1930-1968 extended into Oklahoma
*178: South Carolina, 180 mi, 1932-1937 extended into North Carolina
*280: Georgia, 247 mi, 1931-1954 extended into Alabama
*195: Washington, 95 mi, 1969-1980 extended into Idaho (which it had done at the other end until 1969)
*98: Florida, 169 mi, 1933-1955 extended into Alabama (not a target after 1952 extension)
became other U.S. Routes:
*410: Washington, 469 mi (thus never a target, even if it didn't cross into Idaho), 1926-1967 became 12
*112: Michigan, 207 mi, 1935-1961 became 12
*118: Wisconsin, 35 mi, 1926-1937 became 151
*28: Oregon, 462 mi (thus never a target), 1926-1952 became 26 and 126 (the latter a violator)
*330: Illinois, 161 mi, 1926-1942 became 30 Alternate
*430: Illinois, 37 mi, 1926-1934 became 330 (but that was moved off in 1937 and former 430 became a state route)
*530: Utah, 25 mi, 1926-1938 became 189 and SR 530
*541: Florida, 51 mi, 1931-1951 became 41 and 41 Business
*161: Iowa, 193 mi, 1926-1937 became 151 and 218
*370: Texas, 289 mi, 1926-1939 became 287
*187: Wyoming, 401 mi (thus never a target), 1926-1982 became 191
*94: Florida, 110 mi, 1926-1949 became 41
*96: Texas, 371 mi (thus never a target), 1926-1939 became 59
became state routes:
*104: New York, 167 mi, 1934-1971
*106: Pennsylvania, 95 mi, 1926-1973
*309: Pennsylvania, 214 mi, 1926-1967
*110: Wisconsin, 30 mi, 1926-1938
*210: Minnesota, 127 mi, 1926-1972
*611: Pennsylvania, 131 mi, 1926-1972 (entered New Jersey 1954-65)
*312: Montana, 229 mi, 1962-1979
*213: Maryland, 156 mi, 1926-1973
*120: Pennsylvania, 253 mi, 1926-1967
*320: Wyoming, 26 mi, 1926-1939 (part of US 26 since 1951)
*122: Pennsylvania, 128 mi, 1935-1963
*124: Illinois, 80 mi, 1926-1938
*126: Oregon, 131 mi, 1952-1972
violation*227: Kentucky, 120 mi, 1928-1972
*230: Pennsylvania, 40 mi, 1926-1966
*830: Washington, 209 mi, 1926-1968
*152: Indiana, 176 mi, 1934-1938 (part of US 231 since 1952)
*154: Kansas, 36 mi, 1926-1982
*156: Kansas, 101 mi, 1957-1982
violation*163: Iowa, 60 mi, 1934-1937
*366: New Mexico, 73 mi, 1931-1939
*371: Minnesota, 131 mi, 1931-1971
*189: Utah, 30 mi, 1928-1938
*295: Washington, 45 mi, 1926-1968
*299: California, 295 mi, 1934-1964
*399: California, 136 mi, 1934-1964
So there have been only four actual violations of the rule (as opposed to grandfathered targets).
96 has the same explanation as 126 and 156: when 59 was rerouted and extended, it replaced old 96 southwest of Houston, so they just reused the number on the orphaned piece of old 59.
Why was the minimum threshold set to 300 miles? I mean I know its arbitrary, but I wonder why not 100, 200, 250, etc.
I love how 57 and 96 still exist as they are today, when all you'd have to do to erase the two violations is to swap the numbers. Make 57 into 96 and 96 into 57. Problem solved. They now fit the grid, kinda. The new 57 would be east of 59, at least.