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.

Another fun tidbit is that any 3-digit interstate is an auxiliary route of the 2/1-digit mainline. The first digit determines the type of route, with odd being a spur and even being a bypass or beltway. So I-405 is a bypass of I-5, and I-110 is a spur off I-10 connecting it to the Port of LA.