How US Highways are Numbered
U.S. Highways have been around since the 1920's and the Interstates since 1956. The Interstate system comes from President Dwight Eisenhower. United States highways and interstates are numbered within a nationwide grid.
These highways are sometimes referred to as federal highways but they have always been maintained by state or local governments since their initial designation in 1926. However much of the funding does come from the ferderal government.
In 1956, uniform construction standards were adopted, governing such things as access, speeds, number of lanes, width of lanes and width of shoulders.
Standards were also established for numbering the routes:
* Routes with odd numbers run north-south.
* Routes with even numbers run east-west.
* For north-south routes, the lowest numbers are in the west.
* For east-west routes, the lowest numbers are in the south.
So, I-5 runs north-south along the west coast, while I-10 runs east-west in the south.
When an interstate hits a major urban area, beltways around the city carry a three-digit number. These routes are designated with the number of the main route and an even-numbered prefix. To prevent duplication within a state, prefixes go up. For example, if I-80 runs through three cities in a state, routes around those cities would be I-280, I-480 and I-680. This system is not carried across state lines, so several cities in different states can have a beltway called I-280.
When I-95 hits metropolitan Washington, D.C., coming from the south, it becomes the famous Beltway that circles the city, signed I-495. North of the metro area, when the two circumferential highways rejoin, it becomes I-95 again.
Divided routes have been around since 1926, and designate roughly-equivalent splits of routes. For instance, U.S. Route 11 splits into U.S. Route 11E (east) and U.S. Route 11W (west) in Knoxville, Tennessee, and the routes rejoin in Bristol, Virginia. Occasionally only one of the two routes is suffixed; U.S. Route 6N in Pennsylvania does not rejoin U.S. Route 6 at its west end. The American Association of State Highway and Transportation Officials has been trying to eliminate these since 1934; its current policy is to deny approval of new ones and to eliminate existing ones.