How do individual states come up with there numbering system for there highways?

Odd numbered highways run north and south.





Even numbered highways run east and west.





Although this is a general rule, it is not set in stone.How do individual states come up with there numbering system for there highways?
highways north and south are odd numbered and highways east and west are even numbers....


I beleive this practice is US wide...How do individual states come up with there numbering system for there highways?
Both answers apply to interstate highways. Also for interstate highways they're numbered low in the south to high in the north. And low from the west to high in the east.


Don't know the answer for state and local highways though.