The numbering system can be summarized in these general rules. One- and two-digit highways are national routes:

Odd numbered routes run north-south, and higher numbers are farther east.
     I-5 runs up and down the West Coast, with I-95 on the East Coast.

Even numbered routes run east-west, and higher numbers are farther north.
     I-10 runs along the Southern states, and I-90 runs near the US-Canada border.

Three-digit highways, or Auxiliary Interstate Highways, run only within a single city.

An odd first digit signifies a spur route, which may begin at a large highway and terminate at a city center.
     I-110 travels into downtown Los Angeles.

An even first digit signifies a loop route around a city or a bypass through a city.
     I-215 travels the perimeter of Salt Lake City.
     I-895 serves as a bypass of downtown Baltimore.

The last two digits signify the highway’s origin.
     I-480 eventually returns to I-80, a primary interstate highway.
     I-185 in Georgia leaves I-85 to go into Columbus.

• Did You Know?