u.s. highways have a special way to figure out their numbers. the main u.s. highways usually have numbers with 1 or 2 digits. an exception is u.s. route 101, which is a 3-digit route that is treated like a 2-digit route with a first digit of "10". u.s. highways that run east/west are even numbers (2, 4, 6, 8, or 0), and north/south u.s. highways are odd numbers (1, 3, 5, 7, or 9). for east/west u.s. highways, the small numbers are in the north and the big numbers are in the south. for north/south u.s. highways, the small numbers are in the east and the big numbers are in the west. the important east/west u.s. highways usually have numbers ending in "0" and the important north/south u.s. highways have numbers ending in "1". many of the routes ending in "0" or "1" used to go across the country, but no longer do because of the interstate highway system.