How Are United States Highways Numbered?
Interstates are multilane, divided highways. The main north-south interstate highways always have odd numbers of one or two digits. The system, beginning with Interstate 5 on the West Coast, increases in number as it moves eastward. It ends with Interstate 95 on the East Coast.
The east-west interstate highways have even numbers. The lowest numbered highway begins in Florida with Interstate 4, increasing in number as it moves northward. The highest-numbered east-west interstate is Interstate 96, which begins in Michigan. East-west interstates that run coast-to-coast—such as Routes 10, 40, and 80—end in zero. An interstate with three digits is either a beltway (a roadway around a major metropolitan area) or a spur route (an offshoot of an interstate).
U.S. routes, which are two- or three-lane, undivided highways, follow the same numbering system as the interstate system (even-numbered routes run east-west and odd-numbered routes run north-south). The only difference is that U.S. routes increase in number as they move westward and southward. U.S. Route 1, for example, runs along the East Coast; U.S. Route 2 runs along the Canadian border. U.S. route numbers may have one, two, or three digits.
Sources: Blocksma, Mary. Reading the Numbers, pp. 97-98; Gousha 1996 Road Atlas, 19th ed., p. 2.