America's Interstate System

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Should you find yourself as a motorist in the Continental United States, with a destination somewhere else in the Continental United States, America's Interstate Highways are the best way for you to drive there at a minimum of time and money. Before you get on an interstate, there are a few rules you should know, so you can get approximately where you're headed without a map. (If you have a map, you may still want to read this, is it's rather interesting.)
First, the majority of Interstates occupy land in more than one state, giving them their "Inter" State nature. If they remained entirely within a single state, they would be "Intra" States. However, this is not a hard and fast rule: Some highways in single states received funding under the interstate program, and thus have an interstate designation.
Second, the majority of Interstates are numbered with regard to their direction: Routes with a general North-South direction are oddly-numbered, and East-West routes have even numbers. Numbers are lowest in the South and West.(ie. Interstate 4 in Florida, and Interstate 5 through California et al.)
Third, spur routes, that is, routes which are attached to interstates without necessarily being fully fledged interstates themselves, have a third digit at the beginning. If this digit is even, the route begins and ends at other Interstates. If it is odd, the route has an unattached trailing end.
Fourth, Mile Numbers. Interstate mile numbers start at 0 at the western- or southern-most point, and increase toward the east or north.
Fifth, Exit Numbers. (sigh.) There is no hard and fast rule governing Exit numbers, but individual states choose which of the two following options they wish to employ:
    Numbered by distance: The exit numbers correspond to to the mile numbers. If there is more than one exit in a single mile, letters are affixed to the exit numbers to distinguish them.*Numbered by Quantity: In other words, the first exit is exit 1, and the second exit, 13 miles away, is exit 2. Makes more numerical sense, but new exits require either changing every exit number along the Highway in the entire state, or the addition of letters, which seems okay until exit 3 is 20 miles from exit 3A.

Sixth, Tolls. The majority of Interstate highways are freeways, however some segments are tolled. These include the New York State Thruway, and Alligator Alley, I-75's jaunt from Naples to Fort Lauderdale, Florida.
Exceptions: If an Interstate follows a diagonal route, its number can be chosen as you like it, with due regard to other Interstate numbers.* Interstate 19 is signed in Metric. Interstate 238 is not a spur route of I-38, as I-38 does not exist. * I-99 is west of I-81, and is a spur route of I-80.
A convenient online list of Interstates and their quirks can be found at ZZYZX's Interstate List. Happy Motoring!

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