(last updated 8/21/2020)
"One great red line"
For about a quarter-century, US Route 6 was the longest highway in the country (and it may still be, depending how one measures). 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 6 runs through 14 states, so that leaves lots of room for error. And I suspect there have actually been numerous errors in the past, which might explain why one can find many different figures for the length of US 6. Back when the route still went all the way to Long Beach, an oft-cited figure was 3652 miles. For example, that was the distance used on both a 1953 plaque in Long Beach and on a sign fairly-recently posted in Provincetown:
However, signs that were posted in Provincetown during that actual timeframe gave at least two different figures which were similar to each other but significantly less than 3652:
Kind of interesting to note the "route" illustrated for highway 6 was not very accurate, especially in the West, where it looks like the terminus was in the Bay Area rather than Long Beach. Anyway, was the town of Friend really the midpoint of US 6? From my calculations, it was actually pretty close. The true midpoint was about 12 miles west of Friend, somewhere between the towns of Exeter and Fairmont.
The truncation to Bishop shifted the midpoint of US 6 by 125 miles, and by my calculations it is now located not far from Oakland IA. That truncation is often credited with moving US 20 into the top spot in terms of the longest highway in the U.S. But US 6 is still very close, and in fact may still be longer, depending how one measures.
One final note: the length of US 6 is not a static thing; it has changed many times, and it will change again. For example, in 2019 Iowa DoT changed the routing of US 6 through Council Bluffs, which increased its length by three miles.