Why are the Interstates numbered odd N to S & even E to West. Does it have something to do with land mass?

Question

I was told many years ago that I-95 meant that 5% of land mass was East and 95% land mass was West. Same thing for I-10; 10% of land mass was South and 90% North. I-75 would mean that 25% of land mass was East & 75% West. Is this true? If you look at the routes it makes sense. My information came from an enployee of the Transportation Department.

Answer 1

While that answer may make some sense, it is not correct. The interstates were numbered from west to east and south to north as an orderly method for travelers. Additionally, interstates ending in a "0" or "5" are cross-country routes, such as I-80 crossing the width of the United States or I-35 crossing from Texas to Minnesota.

Another key reason the interstates are numbered west-east is because US highways are numbered from east to west, with US 1 on the East Coast. Planners did not want to confuse travelers by having interstate and US highways with the same route number in the same city. However, in the Midwest, you can find cities were the routes are the same.

Answer 2

If the interstate highways were numbered in that way, you could reverse the system N to S and E to W and get the same result. I don't think that was the principle behind it.
From all I've read, the interstates use the same odd-even numbering system as the federal highways but the high and low numbers are reversed to avoid confusion between the interstates and federal highways. For example, US 1 and 101 run N and S on the E and W coasts but I-95 is E and I-5 is W, and US 2 runs E and W in the north but I-4 is the E-W interstate that is the farthest S.
Other odd facts about interstates: On branches such as I-195 and I-295, an odd first number denotes a spur road into a city, while an even first number denotes a ring road around a city. The reason why there are interstates in Hawai'i is because the reason the system was built in the first place was for national defense; most of the interstates on the island of O'ahu run between the military bases there.

Answer 3

Actually land mass has nothing to do with it. Your example of I-95 proves that it does not. I-95 runs from Maine to Florida on the east coast. None of I-95 is in the west!!

The road numbering system does have a relationship to direction, with odd numbers being primarily north /south routes, and even numbers being east /west. You may find stretches of road that do not follow that rule, due to the geographic layout of the road. Again using the I-95 example, the road goes around Washington DC, rather than through it, so there are sections on the road that run east / west.

Answer 4

This site has tons of information and trivia on the subject of highways:
http://www.us-highways.com/

Included are such things as lists of all the N-S routes and E-W routes, the state they pass through, and their histories (I note by looking at these that the odd-even may not always hold true since occasionally evens have been replaced by odds [and vice versa] in the route's evolution). Here's the link for N-S routes as an example:
http://www.us-highways.com/us1.htm

And, finally, here's their version of an explanation of the numbering system:
http://www.us-highways.com/bus98.htm

In summary: "The U.S. Numbered Highway System is not a true Federal program for roadways, but a series of interconnected state highways with a unified numbering plan for the purpose of aiding navigation. They were chosen initially from higher traffic roads that had previously identified by the states as deserving of Federal Aid. Not every Federal Aid route was given a US number. US highways are built and maintained by the individual states and funded in the same manner as any other state highway. In most states, they are simply a state highway with a different shield. Numbering and signing of US routes is approved by the American Association of State Highway and Transportation Officers (AASHTO). Participation the the US Route System is voluntary. States are not bound by Federal Law to comply with US route designations."

In short, the practice of hwy numbering is a convention that has developed over time without the rule of law.

Answer 5

In 1925, the American Association of State Highway and Transportation Officials set the guidelines for numbering interstates and U.S. highways. Some of these guidelines are:

Interstate north-south routes have odd numbers, with numbers increasing from west to east.
Interstate east-west routes have even numbers, with numbers increasing from south to north.
Interstate highway routes have one- or two- digit numbers.
North-south interstates ending with a 5 and east-west interstates ending with a 0 are typically major cross-country routes.
A three-digit interstate always ends with the two-digit number of the main interstate it loops off from, except I-238.
Three-digit road numbers beginning with an even number are either beltways that go around a city or freeways that go through a city.
Three-digit road numbers beginning with an odd number branch off the main interstate.
U.S. highway north-south routes have odd numbers, with numbers increasing from east to west.
U.S. highway east-west routes have even numbers, with numbers increasing from north to south.
U.S. highway east-west routes ending in 0 tend to be cross-country routes.
Three-digit U.S. routes contain the two digits of their parents routes, but there is not an odd and even number system.
See also: American Association of State Highway and Transportation Officials website: http://www.transportation.org/

Source: Summarized from an article in Via, March/April 2000. p.13.