From the
misc.transport.road FAQ:
There's a
very strict plan for
numbering Interstate routes. The one and two-
digit routes are the
mainlines, with the
even numbered routes going
east-west, and the
odd routes going
north-south. The even routes' numbers
increase as you go north, with the odd Interstate roads increasing to the east. The
major routes end in 0 or 5. As for
three-digit interstates, those that both start and end at a 2-digit route get an even first digit, while those with a
dangling end or ends have an odd first digit. Some
states do this
differently; even 3dis usually return to the parent.
California is an example. The last two digits are the parent 2di.
Q: What happened to
letter extensions on Interstate highways?
A:
AASHTO banned them in 1980 and told state
DOTs to think of other numbers. Two still remain on
I-35.
I-15E in California stayed until 1982.
Q: Why does I-35 split in
Dallas and
Minneapolis?
A: As stated in the
previous question, some Interstates once carried
letter suffixes. All but two were changed in 1980. Those two are the I-35 splits in Dallas and Minneapolis, which remain because there was
no consensus on what the new numbers would be. The smaller cities,
St. Paul and
Fort Worth, did not want their Interstate highways "demoted" to
3di's.
Q:
Why the heck does
I-99 have such a
weird number?
A: This has been one of the most
contentious questions or topics on the
roadgeek newsgroups. Unlike other
interstate highways, which have their numbers assigned by
AASHTO, I-99's number was assigned in a piece of
appropriations legislation sponsored by Rep.
Bud Shuster (R-Pa.), through whose
district the highway runs.
Many of the regulars on
m.t.r. and #
roadgeek take issue with this highway designation because:
1) Its number was
written into law,
as described above;
2) Its number is
out of place--a highway designated as I-99 should run right along the
Atlantic coast; and
3) From its
length (under 70 miles),
location (in an area where all the available
2di designations are already being used), route (basically, a
spur from
I-76 to the
Altoona/
State College area, at least until it connects to
I-80), and the fact that
it doesn't come close to any interstate other than
I-76, an odd 3di designation (such as
I-576) would
be more appropriate.
As a side note, I-99 also has the
distinction of being the only 2 digit interstate that does not have a
direct interchange with any other interstate.
More information is available at http://www.pahighways.com/IHwys/I99.html.
Q: What is
I-238?
A: Interstate 238 is a 2-mile
freeway near
Fremont,
California. It is
not numbered according to the
regular Interstate numbering conventions, rather, it was an
extension of CA 238. There is no
I-38. More information is at http://www.gbcnet.com/roads/I-238/.
Q:
Why are there two Interstates 76, 84, 86 and 88?
A:
All duplicates but 86 arose when most
letter suffixes on Interstates were removed in 1980. Eastern
I-86 was recently added because there were no other choices that would fit. At one time, an eastern I-86 existed from
Hartford, CT to
I-90 in MA; this is now part of
I-84. There was also a duplicate
I-39, but it is now signed along
I-90.