I always knew there was a logic to it, but I never saw it explained so well until I stumbled upon this delightfully informative short video on how the US interstates are numbered.
Those with 2-digits traverse the entire country
If they end in "0" they run East-West (10, 20, 30, ..)
If they end in "5" they run North-South (5, 15, 25, ..)
Those with 3-digits are bypasses and contain the last 2 digits of the interstates they bypass.
That's it! (plus exceptions
) View: https://url.americanpatriotsforum.com/2hbkMS