Known officially as the Dwight D. Eisenhower System of Interstate and Defense Highways, this massive federal road-building project began in the late 1930s. But it wasn't until 1952 that Congress authorized spending and construction began. 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.
The major transcontinental routes are:
|I-10||Los Angeles, CA||Jacksonville, FL||2,459.96 mi|
|I-80||San Francisco, CA||Teaneck, NJ||2,906.77 mi|
|I-90||Seattle, WA||Boston, MA||3,085.27 mi|
|I-5||San Diego, CA||Blaine, WA||1,382.04 mi|
|I-15||San Diego, CA||Sweetgrass, MT||1,436.89 mi|
|I-35||Laredo, TX||Duluth, MN||1,831.43 mi|
|I-55||New Orleans, LA||Chicago, IL||943.69 mi|
|I-65||Mobile, AL||Gary, IN||888.08 mi|
|I-75||Miami, FL||Sault Ste Marie, MI||1,787.49 mi|
|I-95||Miami, FL||Houlton, ME||1,892.76 mi|
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 (as in the political expression "inside the Beltway") that circles the city, signed I-495. North of the metro area, when the two circumferential highways rejoin, it becomes I-95 again.
There is also a system for numbering interstate interchanges, also known as exits. States do this numbering and can choose between two methods:
- The consecutive numbering system starts at the most western or southern point on each interstate route, and interchanges are numbered consecutively (so the first one is interchange #1).
- The milepost system numbers the interchange according to the miles counted, starting at the most western or southern point. An interchange occurring between mileposts 4 and 5 would be designated interchange #4.