Why is it difficult to obtain accurate end-to-end mileage for many US highways?
This comes as a surprise to some, but US highways are actually not federal highways... at least not in the sense that they are owned and maintained by the federal government. Rather, the US routes are actually just state highways... although they are "special" in the sense that at some point they were granted permission by AASHO (later AASHTO) to be signposted with a US route shield.
"One great red line"
For about a quarter-century, US Route 6 was the longest highway in the country. In Jack Kerouac's novel On the Road, the main character described it as "one great red line across America". During those years it ran between Provincetown MA and Long Beach CA, and I believe there has never been a longer highway in the United States. Exactly how long was it? Well, believe it or not, the answer to that question is debatable (see this page for more details about why it is difficult to calculate).
US 264, US 501, US 27, US 104, US 441, Apalachicola, Intra-state routes, Wrong-way overlaps, US 164[i], US 121[i], US 221, Joplin, US 202, US 641, US 136, US 34, US 48 [ii], US 36, US 21, US 219, US 220, Keokuk, Colors, US 171, New Orleans, Jacksonville, US 68, US 150, Philadelphia, US 64, US 11, Erie, Waverly, US 14, US 77, US 83, US 212, US 216, US 385. US 73, US 169