It depends how you measure... and specifically what you measure. If you are familiar with the mileage signs posted at the terminus points of US 6 and US 20, you might point to those and say, "The answer is plain to see: US 20 is 160 miles longer than US 6."
If both of those signs were accurate, then obviously US 20 would be longer. But the fact is, only one of those signs is accurate, whereas the other significantly overstates the true mileage. I make that statement based on measurements I first took in 2016, and later verified in 2020, using a method that I believe to be quite accurate (and in some cases, more accurate than the "official" distances posted by state departments of transportation). So let's take a look at those mileages:
US 50 is one of the longest highways in the U.S. How long is it? Well, there's actually some debate on that (this page has more details about why that is).
Both the Maryland and California state departments of transportation have placed reciprocal mileage signs at the terminus points, each claiming the total distance is 3073 miles. This photo was taken in Ocean City MD, looking along the east beginning of US 50...
(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).