Highway signs tell you more than what road you're on. The numbering of Interstate highways in the United States follows a specific pattern. All signs identifying Interstate highways are blue with red tops and usually have two digits. Interstates that run north and south are odd-numbered. The lowest numbers are on the West Coast, and the numbers go higher as you move east. (I-5, for example, runs from San Diego to Seattle; I-95 stretches from Miami to Bangor, Maine.) Interstates that run east and west are even-numbered. Their numbers are higher in the North. Three-digit numbers indicate that the highway is a connector of a main route.
Source: Digital Marketing Services for MapQuest.com