system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The system for numbering interstate highways.
gpt
Primary routes have two digits. Odd numbered highways run north and south, with lower numbers in the west and higher numbers in the east. Even numbered routes go east-west, with the lower numbers in the south and higher numbers in the north.

Three digit routes starting with an even number are loops within or around a city. Three digit routes with an odd number are spurs into a city. For such three digit routes, the last two digits indicate the primary route they are based on.