A few things you may not have known about Interstate HighwaysBy the Numbers

More than 20 percent of our summer vacation driving this year (1999 - gas was about $1.17/gl) will be on Interstate Highways. That's astounding considering that the 42,000-mile U.S. Interstate Highway system  accounts for less than one percent of the nation's roads. Given the frequency with which American drivers use Interstates, it's also astonishing how little we know about the roads. What's to know, you ask? Well, how about that there's a reason and rhyme to why each Interstate is numbered the way it is? And did you know that, if you understand how the Interstates are numbered, you'll almost never get lost?
Even numbers are always assigned to roads that travel in an east-west direction. That rule actually applies both to Interstate Highways identified by one or two digits--I-80 that traverses the northern part of the country and I-40 in the south, for example--and U.S. Routes, which are identified by black-and-white signs made famous by Route 66.
Odd numbers always represent roads that travel in a north-south direction. This rule encompasses both the Interstate Highways identified by one or two digits--I-5 along the nation's west coast and I-95 along the east coast, for instance--and roads that are part of the U.S. Route system. There are very few exceptions to this even/odd numbering system, but it shouldn't surprise you to learn that there's a website http://www.ihoz.com/viol.html devoted to explaining every deviation.
If an Interstate Highway has three digits, then look carefully at the first number. If a three-digit Interstate starts with even number, then it means the road goes through or around a major metropolitan area. The much-discussed "inside the Beltway" political mentality is actually a reference to Interstate 495, a portion of I-95 that encircles the District of Columbia.
A three-digit Interstate Highway that begins with an odd number indicates the road is a spur that leads directly to a city and does not rejoin the main interstate. Interstate highways 380, 580, and 980, for example, are spurs off I-80 and they lead to cities in the San Francisco Bay area. Once again, don't be surprised by the fact that there's a website http://www.kurumi.com/roads/3di/3di.html   devoted to discussing the quirks and characters of three-digit Interstate Highways. In fact, there's even a website http://gbcnet.com/roads/I-238/ devoted to Interstate 238, the only three-digit Interstate that breaks the rule.
There's one more thing you should know about Interstate Highway System. Those rectangular green-and-white mileage signs also give directional clues. The mile markers are numbered consecutively, beginning at the south (on north-south Interstates) or the west (on east-west routes). So, ascending mile markers on a east-west Interstate mean you are traveling east. Ascending mile markers on a north-south Interstate mean you're traveling north.