U.S. Highways

This is a little trickier. The U.S. route system follows a geographic system similar to that of the interstates, but there are more discrepancies.

For east-west routes:

They are also one- or two-digit even numbers.
The numbers increase north to south.
The extremes are US 2 along the northern border and US 98 in the south.

For north-south routes:

They are also one- and two-digit odd numbers.
They increase from east to west.
The extremes are US 1 on the east coast and US 101 on the west coast.

Major vs. minor routes:

The Interstates have that nice system where a route ending in a 0 or 5 is a major cross-country thoroughfare. You're not so lucky with US highways.
East-west routes that end in a 0 do tend to be cross-country routes (20, 30, 40, 50).
But it's less cut and dry for north-south routes. For example, US 61, 63, and 65 are all major highways that make it close to all they way up and down the Mississippi River corridor. If there was a rule that a highway ending in a 1 or 5 was major, it's been blurred.

For three-digit US routes:

They have the two-digit part of their parent route.
The first digit is given by where in between the grid it falls. For example, if you're numbering three-digit routes between US 61 and US 63, the three-digit routes would be 161, 261, 361, etc.
Since so many US routes have come and gone, you will find three-digit routes that do not touch their parents (i.e. US 138, US 666, US 310).
Rule of thumb is that a 3dus should be on the same orientation as its parent (last digit even = east-west). This generally is true, but there are a few violations, such as US 218 in Minnesota/Iowa and US 310 in Montana/Wyoming. Strange, but I haven't been able to find any east-west odd 3dus routes. It appears only north-south even 3dus routes are violators.
Oddity: US 101 on the West Coast is considered a two-digit route.

AASHTO has the policy that US highways that are only in one state and/or less than 300 miles long should be decommissioned (tuned back to a state number).

Majorly out of place routes include US 52 (because it goes across the country diagonally), US 6 (it falls along US 50 for a while), and US 59 (along the US 71-75 corridor).

The US highways came about from AASHTO in 1924 (back then it was just AASHO). Its Committee on Adminstrration assigned its Subcommittee on Traffic Control and Safety to work on creating some system of numbering all federal aid routes that extended between states. Then, in March 1925, Secretary of Agriculture Gore appointed a Joint Board on Interstate Highways and gave it the job a preparing a numbering system of all federal aid highways and to develop standards to sign them. The board consisted of Thomas H. MacDonald, chief of the Bureau of Public Roads; E. W. James, chief of design for the Bureau of Public Roads; and state highway officials from many states. The final report and recommendations were given to Sec. Gore in October 1925, and he answered with agreement on November 18. The plan outlined the rules we still use today: Even for east-west, odd for north-south, and diagonal routes would have the same number if continued for any great distance. Also, the report from the AASHO subcommittee in 1924 prescribed the standard shapes for signs we still use, as well as the color for traffic signals.

For more on U.S. highways, go to James Sterbenz's US Route list or Robert Droz's Unofficial US Highways Page. Also go to AASHTO's US Highway Sign Policy, from Richard C. Moeur's site.
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