Spur route
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A spur route in the United States Interstate highway system refers to a branch off of a primary interstate that connects with a destination away from the primary interstate. A spur route is distinct from a bypass or a beltway (or loop) in that it does not reconnect with the primary interstate. However, like bypass and loop routes, spur routes have a 3-digit route number in which the last two digits are that of the primary interstate. Spurs can be branches off of other highway types too (e.g. US highways, state routes, etc.).
Interstate Highway Spur Routes
Many Interstate Highways have spur routes when they enter a large metropolitan area. Interstate Spur Routes are numbred with a three-digit number. The last two digits of the number are is the number of the "parent" interstate; e.g. a spur route of Interstate 90 would be x90, a spur route of Interstate 5 would be x05. x is replaced by a number from 1-9, depending on how the intrestate runs.
If the spur route goes around the city or through the entire city, it is given an even number to begin with. However, if the spur route ends somewhere inside the city, it is given an odd number. For example, in the case of Interstate 5's spur routes in Los Angeles, Interstate 405 goes through the entire city, whereas Interstate 105 ends at Los Angeles International Airport.
However, there are many numbering violations in the spur route numbering system, thus the general rules above do not always apply. See List of Numbering Violations of the Interstate System.