A milepost equation, milepoint equation, or postmile equation is a place where mileposts on a linear feature, such as a highway or rail line, fail to increase normally, usually due to realignment or changes in planned alignment.
[1][2] In order to make mileposts consistent with the real mileage, every milepost beyond the equation would need to be moved.
This would usually be caused by a relocation that shortened the distance by 1.6 miles.
It is also possible for an equation to add mileage to what it would otherwise be; the duplicated mileposts receive a special prefix, such as Z.
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