| 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. |