It's a cool system. The numeric rules are (though there are lots of exceptions):
* Primary interstates are one or two digits. Shorter spurs, loops, and connectors are three-digits.
* East-west primary interstates are even-numbered, north-south are odd.
* Major arterials are divisible by five. (The longest interstates are I-90, I-80, I-40, I-10, I-70, I-95, and I-75.)
* Odd routes increase in number going from west to east.
* Even routes increase in number from south to north.
* The last two digits of a three-digit interstate usually identify its main parent interstate. (For example, the I-310 spur connects I-10 to US 90.
* Primary interstates are one or two digits. Shorter spurs, loops, and connectors are three-digits.
* East-west primary interstates are even-numbered, north-south are odd.
* Major arterials are divisible by five. (The longest interstates are I-90, I-80, I-40, I-10, I-70, I-95, and I-75.)
* Odd routes increase in number going from west to east.
* Even routes increase in number from south to north.
* The last two digits of a three-digit interstate usually identify its main parent interstate. (For example, the I-310 spur connects I-10 to US 90.