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:

One of the many interesting things about this segment of US 34 is that it essentially follows the valley of the Colorado River between its headwaters in Rocky Mountain National Park and its confluence with the Fraser River in Granby.  Consequently US 34 also provides access to all of the major water collection components of the Colorado-Big Thompson Project.  Those include Grand Lake (Colorado's largest and deepest natural lake) and three artificial impoundments (Shadow Mountain Lake, Lake Granby, and Willow Creek Reservoir).  Since there are many examples of highways which were originally built parallel to rivers, but which later had to be realigned because of water storage projects that were built in those valleys, I set out to explore whether US 34 had any historic alignments that existed prior to the reservoirs in the Middle Park area.

US 411[i], US 121[i], US 117[i], Wilson, Wilmington NC, Pee Dee, Florence, US 21, US 170, US 421, US 3, Chattanooga, US 72, US 222, US 541, US 411, US 138, US 119, US 522, Brownsville, US 83 history, US 99, US 51, US 63, US 20, Wilmington DE, Cincinnati, US 22, US 124, US 136, US 150, US 152, US 8, US 264, US 501, US 27, US 104, US 441, Apalachicola, Intra-state routes, Wrong-way overlaps

Of the "main" US highways (i.e. the one- and two-digit routes), the longest nine were all east-west routes (6, 20, 50, 30, 40, 60, 70, 80, 12).  And if we take the longest 16 highways, only one of them is a north-south route (US 1, shown in red here):

That stands to reason, because the United States is roughly twice as wide from east to west as it is from north to south.  So it makes sense that there would be more long east-west routes.

From humble beginnings to great lengths

US 83 was among the inaugural routes of 1926.  But at the time, and for the next couple years, it was a very short route -- only about 170 miles long -- running between the capital cities of the two Dakotas:

However, by the early 1930s the US 83 designation had been extended not only north to the Canada border, but all the way down to the Mexico border too.  This increased US 83's distance to nearly 1900 miles, and made it one of the longest north-south US routes.  Additionally, as a result of US 83's new role as a trunk route, three newer US routes were numbered as branches of US 83.  Two of these soon became quite lengthy themselves, and they still exist (US 183 and US 283).  Just as these highways intertwine with each other, their history is also interrelated.  This article examines the evolution of US 83, as well as the x83 routes that are part of its "family".

We will begin with the following map, which illustrates the current routings of US highways 136 and 150 between Indianapolis and Bloomington.  From its eastern terminus in Indianapolis, US 136 (the blue line) heads west to Danville.  US 150 (the red line) approaches Danville from the south.  The two routes "bump" there before separating again, and then they have another intersection just south of the town of LeRoy IL:

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...

Many Route 66 tourists simply enjoy the romance and the lore of the fabled road.  The facts aren't all that important, and if one of the legends turns out to be a tall-tale, they'd rather not know about it.  And that's fine -- if you're one of those people, I say you gotta do you -- get out there and enjoy the Mother Road.

...then you might want to stop reading here, because this page contains research that discredits some of the "conventional wisdom" about US 66 that is found elsewhere.  On the other hand, if you're interested in facts and details, then please carry on.

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. ​

(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).