The Logic To American Interstate Highway Numbering

• Odd numbers are North-South routes. Numbers increase west-to-east, original backbone routes are on the 5s. (ex. I-5, I-95)
• Even numbers are East-West routes. Numbers increase south-to-north, original backbone routes are on the 10s (ex. I-10, I-80)
• Three-digit numbers with even first digit are loops that connect to their parent Interstate (the last two digits) twice, to avoid cities or serve an alternate route (ex. I-405 in LA, I-276 in Philadelphia)
• Three-digit numbers with odd first digit are spurs that cross their parent Interstate (the last two digits) once, though they may head off in both directions

Thanks, Dwight Eisenhower!