Last original Interstate built?

[Walleye2013]

I have an old Rand McNally with I-69 in Michigan--but there is a parallel road that presumably people used while it was built. However, the road is grey (no US or local route number attached to it).

Are you talking about near Port Huron?  That would probably be Lapeer Road which used to be M-21.

Northeast of Lansing is Lansing Road which used to be M-78.  There is also a Lansing Road southwest of Lansing toward Charlotte which used to be US-27.  Both of these served as TEMP I-69 for several years. 

Some of these may remain as unsigned state routes; Michigan sometimes does that when a freeway replaces an old road that gets decommissioned.

I think those were them--on the '80 Rand McNally I have it shows the "grey" routes. Also, random question: Do state routes have to end at another state route or can they just end randomly? OH-174 starts in Willoughby but ends at Old Mill Road in Gates Mills--rather than another US or OH route.

OH-174 ends at Old Mill Road because Old Mill Road used to be US 322 before the "bypass" of Gates Mills was built.

[Avalanchez71]

So what is the conclusion to this question as posed by the astute NE2?