71

[SeriesE]

I believe the only reason those freeways got a 2 digit number is that they cross state lines, hence being an actual interstate so 3dis aren't appropriate.

Then why is I-395 in MA and CT a 3di and not a 2di? It crosses a state line and is a direct corridor.

There is an exception to every rule. This is why we have I-238.
It's really up to the whims of politicians who approved or requested the number: whether they need the votes or not.

[epzik8]

Also, I-97: two digits 'cause why not!

It connects Baltimore with Annapolis and the Bay Bridge via US 50, serves as part of the alternate route to Richmond that includes MD 3 and US 301, and passes right by BWI.

By that logic, I-196 in MI, both I-155's in IL and MO/TN, I-380 in IA, I-580 in CA, I-335 in KS, and probably several other 3DIs should be two digits. All are just as useful with multiple local purposes and have destinations well beyond their endpoints. Ben114 stated it more succinctly than I just did, but he's right. The only apparent reason is "why not?"

Yes, why not!  I love little anomalies like that.  Why does I-17 not start at mile 0 (there is a historical reason I know, but a lot of time has passed to correct it and Arizona just won't)?  Why does I-19 have kilometer markers? These things are dumb, but they are kinda cool too!

Delaware Route 1's exits are kilometer-based too. Evidently, both I-19 and DE 1 were constructed when the US was considering converting to the metric system, and this included miles to kilometers. Sorry I'm late on this response.