Thank you all for the thorough replies.
Is there some standard to numbering conventions that is determinant on direction (North/South) or some other metric that makes making each interstate uniquely numbered.
Odd numbers from 5-99 (there is no 1 or 3) are north-south and increase going east.
Even numbers from 2-96 (there is no 98) are east-west and increase going north.
3-digit Interstates can have any posted direction. Those with an odd first digit are spurs, and those with an even first digit are loops.
3-digit Interstates that connect two different Interstates can be given either even or odd first digits.
Multiples of 5 are generally more important than others.
Generally, if an Interstate is 100 miles or more, it will have 2 digits, while those below 100 miles will have 3 digits.
There are several Interstates that break these rules, some more blatant than others.
Examples: Massachusetts has 84, 90, 91, 93, 95, 190, 290, 291, 391, 195, 295, 395, and 495.
I-84 is east-west and south of 90.
I-90 is also east-west, and since it is a multiple 5, it is "major", so it goes all the way to Seattle from Boston.
I-91, I-93, and I-95 are north-south. For the most part (I-93 and I-95 cross), numbers increase going east. I-95 goes from Maine to Florida, while 91 and 93 are nowhere near as long.
I-190 (odd first digit) has I-290 at one end without another Interstate at another end. This could be considered a minor violation for not connecting directly to I-90.
I-290 (even first digit) connects I-90 to I-495, being numbered off of I-90.
I-291 connects I-90 to I-91.
I-391 starts at I-91 and ends at a surface road.
I-195 goes from I-95 in Providence RI to near Cape Cod.
I-295 is a half-beltway of Providence RI.
I-395 connects I-95 in New London to I-90 near Worcester.
I-495 is a half-beltway of Boston MA. It is over 100 miles, but it would make no sense having 2 digits as it is clearly a half-beltway.