Interstate Highway System
North-south interstates have odd numbers. The numbers go up from west to east. East-west interstates have even numbers. The numbers go up from south to north. One and two digit freeways go long distances. Three digit freeways go into or around cities. Freeways that go into cites are called spur routes and start with an odd number. Freeways that go around cities are called loop routes and start with an even number. The last two digits in a spur or a loop route are the same as the freeway it started from.