With the many interstate highways criss-crossing through the cities and countrysides of the U.S., it can seem somewhat baffling as to where they are all going. Why are they given certain numbers versus some other number? Is there any rhyme or reason to it? Indeed there is.

Before we look into that, a brief intro to the U.S. Interstate Highway system:

Its full name is the Dwight D. Eisenhower National System of Interstate and Defense Highways. Named after the champion of its formation and 34th President of the United States, the original portion began construction in 1956 and was expected to see completion in ten years. In fact, it would not see completion until 1991, thirty-five years later. At the time it was the largest public works project in American history, though today its 47,856 miles (77,016 km) is second longest in the world, after China. The total construction cost is estimated at roughly $485 billion.

The American Association of State Highway and Transportation Officials (AASHTO) is responsible for the numbering scheme of the Interstate Highway System. The scheme was first developed in 1957, although the present policy dates back to 1973.

Let's see how this is laid out:

Primary routes of the Interstate Highway System:

East-west highways:

• Assigned even numbers
• Increase from south to north
• Major arteries divisible by 5 (I-10, I-40, I-70, etc.)

North-south highways:

• Assigned odd numbers
• Increase from west to east
• Major arteries divisible by five (I-5, I-15, I-35, etc.)

The directional numbering holds true even if the route temporarily runs against its designated orientation. Major arteries are designated by numbers divisible by five, and are intended to carry traffic long distances. The major north-south arteries begin along the west coast with I-5, running between Canada and Mexico, and continue on to the east coast, where I-95 runs between Canada and Miami, Florida. Likewise, the major east-west arteries begin in the south with I-10, running between California and Florida, up to the north, where I-90 runs between Washington and Massachusetts.

Auxiliary Interstates:

• Three-digit route numbers
• Consists of a single digit prefixed to the two-digit number of its parent Interstate Highway (for example, I-85 runs through Atlanta, while I-285 circumnavigates Atlanta entirely.)

Bypass:

• Deviates and returns from parent without forming a loop
• Even first digit (ie I-220)

Beltway or Loop:

• Deviates from parent, returns and forms loop
• Also even first digit (ie I-285)

Spur:

• Deviates from parent and does not return
• Odd first digit (ie I-575)

Special interstate highways:

Business routes:

• Same designation as the routes they parallel.
• Use a green shield instead of usual red and blue.

These are usually either loops or spurs and travel through the corporate limits of a city. They are needed when the regular route is directed around a city.

Hopefully this helped shed a little light on how the U.S. interstate highway system is configured. Knowing a few simple rules can make it easier to get around and take a lot of the mystery out of traveling out of state or from overseas.