US 20 is the longest highway in the country. 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. So when that distance was referenced in 2016, it 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 somewhere between 55-70 miles less than what is stated on the signs. (That 15-mile uncertainty results from the fact that US 20 does not exist in Yellowstone National Park, so a cross-country traveler must choose between a couple different implied routes, one of which is 15 miles longer than the other; more details in the notes that follow).
Here 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:
155 - Massachusetts 372 - New York 45.3 - Pennsylvania 261 - Ohio 156 - Indiana 199 - Illinois 300 - Iowa 431 - Nebraska 434 - Wyoming [note 1] 79.9 - National Park Service [note 2] 9.7 - Montana [note 3] 403 - Idaho 450 - Oregon 3296 - TOTAL [note 4]
Notes:  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.
 This is the mileage under the jurisdiction of the National Park Service, i.e. the mileage 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.
 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.  US 20 is not signposted within Yellowstone, and technically does not exist inside the Park boundaries. However, in order to calculate the cross-country distance along US 20, mileage through the National Park must be included, via some route that connects the east entrance and West Yellowstone. The shortest route is via the Norris-Canyon Road, which is 79.9 miles (as measured between the aforementioned control points). The preceding mileage chart uses this figure. However, rather than using Norris-Canyon, some maps seem to suggest that US 20 traffic should instead use the south half of the Grand Loop. This distance is 93.7 miles (between the same waypoints referenced above). This is 13.8 miles more than the Norris-Canyon route, which would then yield a total cross-country figure of 3310 miles.