Question:

Interstate Highways: Why are there TWO I-84's?
The eastern one begins in Scranton, PA. and ends in Sturbridge, MA. The western one begins in Portland, OR. and ends in Echo, UT.. Supposedly the Interstate system is finished, and yes, you could get from the eastern end of the western section to the western end of the eastern section (got that?) via I-90 to I-81 but numbering wise it is not a contiguous highway as there's a roughly 2040 mile "detour". Anyone know the reason for this?

Answers:

A-Best: There are also two I-86s and two I-88s. And at one time there was going to be two I-82s. There weren't enough numbers in the system between I-80 and I-90. So since each section is over 1500 miles from the other, they didn't see it as a big problem. And the interstate system isn't finished yet. I-95 still isn't finished. As you come north through PA and NJ, I-95 ends just north of Trenton and the rest of I-95 is over on the NJ Turnpike north of Exit 6. The problem won't be fixed until later this decade.  

A: The reason that they do not have different numbers, as well as the reason that there are two I-86's and two I-88's, is that there are not enough two-digit even numbers between 80 and 90 for all the west-east Interstates that are north of I-80 and and south of I-90 to have a unique number. The reason that the two I-84's are not contiguous is probably that it did not make economic sense to build a third west-east Interstate through Wyoming (80 and 90 both already go through Wyoming), when it is so sparsely populated that there would not be many drivers to use it.