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You are here: Home / blogosphere / How the Interstates got their numbers

How the Interstates got their numbers

08-Jan-2005 By Jim

Cory Doctorow:
CoolGov uncovered this US Highway Administration document that explains the numbering scheme behind the US interstate highway system.

* Major interstates routes have a one or two digit number associated with them. North-south routes have odd numbers (I-5) while east-west roads have even numbers (I-10).

* Connecting interstate routes or beltway loops around urban areas have 3 digit numbers (the 101).

* To prevent duplication within a state, a progression of prefixes is used for the three-digit numbers. For example, if I-80 runs through three cities in a state, circumferential routes around these cities would be numbered as I-280, I-480, and I-680.

* There�s no set standard on exit numbering, but states generally use one of two systems:
1. Milepost numbering. The southern or western-most point on a given interstate begins the odometer at 0. If an exit is 6.5 miles from that point, it�s exit #6 and so forth.

2. Consecutive numbering. Again, starting at the western or southern-most point, each exit is given a number, starting with 1. When they have to shoehorn more exits in, they become #6A, #6B, etc�

ttp://coolgov.com/”>Cool Gov)
…excerpt from: www.boingboing.net…

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