Off the top of my head, I write this. If I find official or specific info, I'll amend it.
As a trend, though probably not strictly adhered anymore.
Major Interstates have two digits.
Even number is east-west. Odd is north-south.
A hundreds digit makes a local sister highway.
A hundreds digit of 1 is a local spur into a city.
A hundreds digit of 2 is an express through a city.
A hundreds digit of 3 is a bypass between highways.
A hundreds digit of 4 is an express around a city.
A hundreds digit of 5 is a local spur into a city.
A hundreds digit of 6 is a wide bypass around a metropolitan region.
A hundreds digit of 7 is a wide bypass around a metropolitan region.
A hundreds digit of 8 is an express through a metropolitan region.
On a larger scale, the middle digit gets larger as you go east, or north. That is, I10 is very southern and I90 is very northern; I5 is very western and I85 is very eastern.
The San Francisco Bay Area has: I80, I280, I380, I480, I580, I680, I880, I980. Most metro areas have at least an I4xx. Dallas and Ft. Worth have a special split: I35E and I35W allow the I35 to visit both cities.