Known officially as the Dwight D. Eisenhower System of Interstate and Defense Highways the federal road building project began in the late 1930's. The congressional authorization for spending did not come until 1952 and construction began in 1952. Uniform construction standards were adopted in 1956. These standards cover items such 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 east.

Example: I-5 runs north-south along the west coast and 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 with I-280.

EXITS - states can choose between two methods for numbering exits.

1. 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 exit #1).
2. The mile post system numbers the interchange according to the miles counted, starting at the most western or southern point. An exit between mileposts 4 and 5 will be exit #4.

Click here for a few fun facts about the Interstate System.