United States of America
|United States of America|
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The Interstate network is the American Equivalent of the motorway network, though there are lots of differences.
Major, 2-digit, Interstates are numbered in a grid - east-west Interstates have even numbers and north-south Interstates have odd numbers. Numbers increase from the SW to the NE. I-x0 and I-x5 are major Interstates, with other Interstates filling in the gaps. There's a shortage of numbers in the north east, leading to duplicate numbers and out of place numbers.
3-digit interstates are spurs and loops off the main network. The last two digits are numbered for the parent interstate. Odd numbers are spurs, even are loops. There are a lot more exceptions to these rules, compared with 2-digit interstates.
All Interstate numbers must be approved on a Federal level. Future Interstates and Interstate Business loops may be signed where Interstates are being constructed and where Interstates have bypassed a town that is full of services for passing traffic.
Interstates cannot have the same number as any US route in the states it travels in. There should also be little conflict between Interstates and state routes of the same number (these can be extensions of the Interstate's route though). The US route/Interstate number rule has recently lost its strictness - US74 and I-74 both exit in North Carolina, and multiplex for a brief while. US41 in Wisconsin is planned to have its Milwaukee to Green Bay section upgraded to I-41 (which fits the grid perfectly as it's between I-39 and I-43).
Mileage and Junction numbering
All mileage posts comes from the south-eastern most corner of the state. Most states have mileage-based junction numbering, though some have sequential (and others are in the process of changing). Junction numbers could be on Interstates only, or all high-quality dual carriageways.