How long is US highway 20, really?

(latest revision 8/31/2020)
US 20 is often cited as the longest highway in the country (and that may be correct, although its main challenger is a lot closer than most people realize).  In late 2016, officials from both endpoints (Oregon and Massachusetts) posted twin signs, one at each terminus of the highway (photos and more info on the US 20 page).  While that was a really great idea, it is unfortunate that the distance listed on those signs (3365 miles) is off by a non-trivial amount.

Wayne Brunelle, Nov. 2016

The source for the 3365-mile figure was the AASHTO route log, published in 1989.  But when one actually looks at that log, one finds that in order to reach the cited 3365-mile figure, one must include not only the mileage for mainline US 20, but also the mileages for all of US 20's auxiliary routes (that is, each of the bannered routes with designations like Alternate US 20 and Business US 20).  But obviously those auxiliary mileages must be subtracted out in order to arrive at an accurate total for mainline US 20, which according to the route log would have been only 3237 miles.

​However, when US 20's mileage was referenced in 2016, the AASHTO route log was at least 27 years old, possibly more.  That raised a red flag in my mind, so I checked it out myself and found that the actual mileage is now even less than 3237.  Why would the mileage change after 1989?  Because during those 30-some intervening years, the route of US 20 has been realigned in several places (perhaps most significantly through Iowa, where it is now a freeway across most of that state).  Every time something like that happens, the mileage of US 20 changes.  As of 2020, when adding up recent data from each of the states that US 20 passes through, that total is closer to 3220 miles.

However, I am not convinced even that figure is accurate.  I checked it out with Google Maps, and following are the figures I used in order to come up with my total.*  These mileages are separated by state and/or jurisdiction, which makes it more convenient to compare against the figures listed in the official route logs from each state dept. of transportation:

​153.1 - Massachusetts
371.2 - New York
45.2 - Pennsylvania
260.3 - Ohio
155.4 - Indiana
197.9 - Illinois
300.1 - Iowa
430.5 - Nebraska
432.8 - Wyoming [note 1]
9.7 - Montana [note 2]
401.0 - Idaho
446.1 - Oregon
3203.3 - TOTAL [note 3]

80.1 - National Park Service via Norris Canyon [note 4]
93.8 - National Park Service via South Loop [note 4]

*I should note that some in the highway hobbyist community of which I am a part have disagreed with this practice, arguing that official DOT figures are more reliable than original research using Google maps.  In most cases they are probably right, although I have seen enough mistakes made by DOTs that I am convinced their data cannot always be taken at face value, at least not without a little fact-checking.  

Notes:
[1] This is the mileage under the jurisdiction of Wyoming DOT, i.e. the mileage between the Nebraska state line and the east entrance to Yellowstone National Park.  This figure does not include the mileage between Yellowstone's east entrance and the Montana state line.  Rather, that mileage is included in the figure for National Park Service.

[2] This is the mileage under the jurisdiction of Montana DOT, i.e. the mileage between the Idaho state line and Boundary Street in the town of West Yellowstone.  This figure does not include the 2.1 miles between Boundary Street and the Wyoming state line.  Rather, that mileage is included in the figure for National Park Service.

[3] US 20 is not signposted within Yellowstone, and technically it does not exist inside the Park boundaries.  So this figure represents the total mileage that is officially designated as US 20. 

[4] However, in order to actually drive US 20 from end-to-end, one must pass through the National Park via some route that connects the east entrance and West Yellowstone.  So for a cross-country trip, we would have to add the mileage under the jurisdiction of the National Park Service.  In other words, the distance between the east entrance to Yellowstone National Park and Boundary Street in the town of West Yellowstone (this figure includes 2.1 miles within Montana, while the remaining mileage is in Wyoming).  The shortest route is via the Norris-Canyon Road, which adds 80.1 miles (as measured between the aforementioned control points).  This would result in a cross-country distance of 3283.4 miles.  However, rather than using Norris-Canyon, some maps suggest that US 20 traffic should instead use the south half of the Grand Loop.  That distance is 93.8 miles (between the same waypoints referenced above).  That is 13.7 miles more than the Norris-Canyon route, which would then yield a total cross-country figure of 3297.1 miles.

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So even if we choose to add the cross-country mileage, and we choose the longer route through Yellowstone, we are still 68 miles short of the 3365-mile figure listed on the signs.  And one could argue that the mileage through Yellowstone should not be added at all, which then calls into question whether US 20 is actually the longest highway.

The midway-point along US 20 is in Nebraska.  The exact location is dependent on how one counts mileage through Yellowstone.  Half of 3310 is 1655 miles, so the midpoint of US 20 would be between the towns of Newport and Bassett.  If we go with the shorter distance of 3296, that would shift the midpoint 6.9 miles further west, which is essentially right in Bassett.

Additional research: Mike Roberson