The route numbers and locations are coordinated by the American Association of State Highway and Transportation Officials (AASHTO).

[4] The only federal involvement in AASHTO is a nonvoting seat for the United States Department of Transportation.

Generally, most north-to-south highways are odd-numbered, with the lowest numbers in the east and the highest in the west, while east-to-west highways are typically even-numbered, with the lowest numbers in the north, and the highest in the south, though the grid guidelines are not rigidly followed, and many exceptions exist.

[4] As of 1989,[update] the United States Numbered Highways system had a total length of 157,724 miles (253,832 km).

"[4] U.S. Route 3 (US 3) meets this obligation; in New Hampshire, it does not follow tolled portions of the Everett Turnpike.

Even numbers tend to increase from north to south; US 2 closely follows the Canadian border, and US 98 hugs the Gulf Coast.

The longest routes connecting major cities are generally numbered to end in a 1 or a 0;[7] however, extensions and truncations have made this distinction largely meaningless.

Many Canadian highways were renumbered in the 1940s and 1950s to adopt the same number as the U.S. Route they connected to – mostly in the western provinces.

Route numbers are displayed on a distinctively-shaped white shield with large black numerals in the center.

Individual states may use cut-out or rectangular designs, some have black outlines, and California prints the letters "US" above the numerals.

Third, they can be displayed on large green guide signs that indicate upcoming interchanges on freeways and expressways.

For instance, US 11 splits into US 11E (east) and US 11W (west) in Bristol, Virginia, and the routes rejoin in Knoxville, Tennessee.

[f] The official route log, last published by AASHTO in 1989, has been named United States Numbered Highways since its initial publication in 1926.

[16][17] In 1903, Horatio Nelson Jackson became the first documented person to drive an automobile from San Francisco to New York using only a connection of dirt roads, cow paths, and railroad beds.

His journey, covered by the press, became a national sensation and called for a system of long-distance roads.

[18] In the early 1910s, auto trail organizations—most prominently the Lincoln Highway—began to spring up, marking and promoting routes for the new recreation of long-distance automobile travel.

[1] Secretary Howard M. Gore appointed the Joint Board on Interstate Highways, as recommended by AASHO, on March 2, 1925.

They decided that the system would not be limited to the federal-aid network; if the best route did not receive federal funds, it would still be included.

The associations finally settled on a general agreement with the numbering plans, as named trails would still be included.

The tentative system added up to 81,000 miles (130,000 km), 2.8% of the public road mileage at the time.

The group adopted the shield, with few modifications from the original sketch, at that meeting, as well as the decision to number rather than name the routes.

The skeleton of the numbering plan was suggested on August 27 by Edwin Warley James of the BPR, who matched parity to direction, and laid out a rough grid.

The five-man committee met September 25, and submitted the final report to the Joint Board secretary on October 26.

[11] The new system was both praised and criticized by local newspapers, often depending on whether that city was connected to a major route.

Because US 20 seemed indirect, passing through Yellowstone National Park, Idaho and Oregon requested that US 30 be swapped with US 20 to the Pacific coast.

[1] Many local disputes arose related to the committee's choices between designation of two roughly equal parallel routes, which were often competing auto trails.

The final solution resulted in the assignment of US 66 to the Chicago-Los Angeles portion of the US highway, which did not end in zero, but was still seen as a satisfyingly round number.

New additions to the system must serve more than one state and "substantially meet the current AASHTO design standards".

[30] The original major transcontinental routes in 1925, along with the auto trails which they roughly replaced, were as follows:[1][11]

US 60's violation of two of the conventions would prove to be one of the major sticking points; US 60 eventually was designated as US 66 in 1926, and later it became a part of popular culture.