Re: If the Cosmosphere were on I70....
It's very simple:
The two-digit U.S. Routes follow a simple grid, in which odd-numbered routes run generally north to south and even-numbered routes run generally east to west. (US 101 is considered a two-digit route, its "first digit" being 10.) The numbering pattern for U.S. Routes is the reverse of that for the Interstate Highway numbers—U.S. Routes proceed from low even numbers in the north to high even numbers in the south, and from low odd numbers in the east to high odd numbers in the west. Numbers ending in 0 or 1 (and US 2), and to a lesser extent in 5, were considered main routes in the early numbering, but extensions and truncations have made this distinction largely meaningless. For example, US 6 was until 1964 the longest route (that distinction now belongs to US 20). The Interstate Highway System's numbering grid, which has numbers increasing from west-to-east and south-to-north, is intentionally opposite from the U.S. grid, to keep identically numbered routes apart and to keep them from being confused with one another.
Three-digit numbers are assigned to spurs of two-digit routes. US 201, for example, splits from US 1 at Brunswick, Maine, and runs north to Canada. Not all spurs travel in the same direction as their "parents"; some are only connected to their "parents" by other spurs, or not at all, instead only traveling near their "parents". As originally assigned, the first digit of the spurs increased from north to south and east to west along the "parent"; for example, US 60 junctioned, from east to west, US 160 in Missouri, US 260 in Oklahoma, US 360 in Texas, and US 460 and US 560 in New Mexico. As with the two-digit routes, three-digit routes have been added, removed, extended and shortened; the "parent-child" relationship is not always present. Several spurs of the decommissioned US 66 still exist, and US 191 travels from border to border, while US 91 has been largely replaced by Interstate 15 (I-15).
Several routes approved since 1980 do not follow the numbering pattern:
US 400, approved in 1994, has no "parent" since there is no US 0 or US 100.
US 412, approved c. 1982, is nowhere near US 12.
US 425, approved in 1989, is nowhere near US 25.
In addition, US 163, designated in 1970, is nowhere near US 63. The short US 57, approved c. 1970, connects to Federal Highway 57 in Mexico, and lies west of former US 81.
While AASHTO guidelines specifically prohibit Interstate Highways and U.S. Routes from sharing a number within the same state (which is why there are no Interstates 50 or 60), the initial Interstate numbering approved in 1958 violated this with I-24 and US 24 in Illinois and I-40, I-80, US 40 and US 80 in California (US 40 and US 80 were removed from California in its 1964 renumbering). Some recent and proposed Interstates, some of them out-of-place in the grid, also violate this: I-41 and US 41 in Wisconsin (which will run concurrently), I-49 and US 49 in Arkansas, I-69 and US 69 in Texas, and I-74 and US 74 in North Carolina (which run concurrently).
Some two-digit numbers have never been applied to any U.S. Route, including 39, 47, 86 and 88.