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.
