States in which you have been on an Interstate/US but not state highway

[US 89]

^ All of I-95 in Georgia is concurrent with the unsigned GA 405.

By the way, this unsigned state route concurrency business applies to every interstate in Georgia and Florida, and to US highways in some more southeastern states. The state route concurrencies on US highways are even signed in GA.

[SSOWorld]

None. I've made it a point to drive at least a few blocks of one in each state in recent trips to check out signage.  It ranges from concurrent with aux/US/Interstate (WY) to full fledge finish of a state route system (WI)

As far as multiplexing with business (Interstate/US) routes - the state route's the main route, the aux route is coincidental (My opinion, YMMV).

^ All of I-95 in Georgia is concurrent with the unsigned GA 405.

By the way, this unsigned state route concurrency business applies to every interstate in Georgia and Florida, and to US highways in some more southeastern states. The state route concurrencies on US highways are even signed in GA.

I don't believe in hidden state routes (though YMMV) - I have to see a sign.

[kirbykart]

Vermont, New Jersey, and Maryland.

[epzik8]

GA: I-95 (GA 405 does not count)
ID: I-15, US 20
IL: I-55, I-70, US 67
IN: I-70, US 40
NE: I-129, US 20, US 75, US 77
RI: I-95, I-295
WY: I-90, US 20, US 89