I was looking at a map of the US that had on it the Interstate Highway System. I happen to be looking at East-West Interstates that cross most, if not all of the US. For example, you can jump on Interstate 80 in San Francisco, CA and drive all the way to Teaneck, NJ, which is a little over 2,900 miles.
As I was looking at the map I noticed something. If you start in North Dakota, and work your way south to Texas you find a number of E/W Interstates that span the country; I-90, I-80, I-70, I-40, and I-10. What you don’t find is I-60, I-50, I-30 and I-20? Were they ever planned, or is the numbering systems the way it is for a specific reason?
Okay, there is an I-20, but it starts in West Texas and heads east from there.
When AASHO developed the new Interstate numbering scheme, it wanted to avoid confusion with the existing interstate numbering system (the one we now think of as US routes). So they skipped I-50 and I-60, which could come have come uncomfortably close to (or even be multiplexed with) US 50 and US 60.
Diagonals have always presented something of a problem for the integrity of the system. I-30 and I-44 were in the original scheme, but the eventual crisscrossing of I-45 and I-49 with I-69 will be a bit of a kludge. Some of the Aspergers-afflicted roadgeeks who post at aaroads.com get hot and bothered by any flaws in the system. Pennsylvania’s out-of-sequence I-99 makes them chew their tongues.
Avoiding repeated numbers is also why the two systems were arranged differently. While both US routes and interstates have east-west routes as even numbers and north-south routes as odd numbers, in the US routes system, the lowest even numbers are in the north, and in the interstate system, the lowest even numbers are in the south. Similarly, I-5 is in the west, and US 5 is in the east.
However, the rule that US route and interstate numbers should not repeat in a state has weakened over time. US/I-24 has been an exception in Illinois since the beginning, but now there’s US/I-41 in Wisconsin, and US/I-74 in North Carolina.
Others have noted that I-20 and I-30 exist, but you are right that they are not cross-continental routes. On the north-south side of things, I-45 exists, but it is not a major route either, covering only a short distance in Texas.
Basically, yes. Technically, both end at the junction with 101. And you are changing directions because you are going around the south end of the bay to proceed up the other side.
Over by Berkeley you can find yourself travelling simultaneously on 80 East and 580 West while actually going north. That pesky bay again.
Keep in mind that the country is wider than it is tall. So there is no need to have nine east-west cross-country routes just because that’s how many two-digit multiples of 10 there are. Assuming a similar spacing as for major north-south routes, it’s inevitable that some would be skipped, or would be shorter than cross-country.
Powers &8^]
The proposed Interstate 270 in Illinois had to be renamed when everyone realized that the beltway would actually intersect with itself, leading to the junction of I-270 and I-270.
The north-south leg was renamed I-255. Nowadays I-255 runs into Illinois Route 255.
Interstates ending with 0 and 5 were designated as the primary cross country routes. There are only ten of each number. But it was not deemed necessary to have ten going east-west, so not all the available numbers were used.
For example, there are only seven Interstates crossing the boundary between the Pacific and Mountain time zones. One of them is I-8. So not all the numbers ending with zero are used.
That wasn’t originally out of numerical order. I-84 used to be I-80N (and I-80 between Salt Lake City and the Bay area was I-80S). They decided to get rid of the N/S designations and renumbered I-80N. There is no even number between 80 and 82, so they ended up with out-of-sequence numbers. What they should have done is renumber I-80S, but then they’d also have had to renumber various side freeways in the Bay Area. So they took the easier way out and renumbered I-80N.
Two others are I-84 and I-15. There’s only 4 interstates that cross the entire US from east to west: I-10, I-40, I-80, and I-90.
I-255 is a ring road applying to traffic N/S bound from I-55, and I-270 pertains to traffic E/W bound from I-70… So it probably reduces confusion, rather than adding to it, since this conforms the rule of even numbers E/W and odd numbers N/S and the ring road is recognized as an alternate for both…
Yes. Strictly speaking, just to the west of Glen Carbon.
I-270 (the part from I-55/I-70 west across the Mississippi) was built and named first. And if they named it all I-255, then we would have ended up with the Intersection of I-255 and I-255.
You think that’s strange. Originally, I-270 was the part of the beltway that ran between I-55/I-70 in Illinois to the intersection with I-70 in Missouri. From I-70 to I-44, the beltway was I-244, and from I-44 to I-55 it was I-255. That turned out to be totally confusing for drivers who were trying to use it to detour around downtown St. Louis, and it finally got one continuous designation.
That would be entirely logical, except that I-270 is marked N/S from I-55 to I-70, then E/W from I-70 on one end to I-55/I-70 on the other. To be fair, it does turn from N/S to E/W.
Good point. While obviously an interstate highway can’t be expected to go all the way to the shoreline, I’d think that to qualify as a true E-W cross-country route, it should end somewhere in a city or town on the Atlantic or Pacific coast.
