A variation on the "longest stretch of interstate where you've used every interchange".
Which interstate interchanges in your state have you used/not used? If an interchange has parts (A/B), it counts as a single interchange and you only have to have used one part to count it as used.
Indiana:
I-64: 19/19, all
I-65: 68/72, all except 115, 116, 117, 119
I-69: 58/68, all except 3, 115, 116, 123, 125, 134, 299, 312, 326, 350
I-70: 30/37, all except 1, 3, 51, 77, 87, 131, 145
I-74: 35/43, all except 220(IL), 8, 99, 101, 103, 109, 123, 143
I-80: 25/25 , all
I-90: 20/20 , all
I-94: 17/17 , all
I-265: 5/5, all
I-275: 1/1, all
I-465: 30/30, all
I-469: 4/14, all except 1, 2, 9, 13, 15, 17, 29
I-865: 2/2, all
Too many to list, especially in the "not used" category on I-81. I think once you get beyond Christiansburg, the only exit I have used on I-81 in Virginia is to go south on I-77.
You have got around. Indiana is the interstate capital of the world.
I'd have a lot of missed entries if I did all of Washington, so I'm limiting myself to the Seattle MSA / tri-county area:
I-5: 118, 120-122B, 125-127, 130-215
I-5 express lanes: All
I-90: 1 to 20, 25, 31, 34, 52 to 54
I-405: 1 to 7, 10 to 30
I-705: All
I-25: 98/147, including all between 181 and 223
I-70: 103/134, including all between 169 and 295
I-76: 28/48
I-225: 11/11
I-270: 6/6
Total: 246/346, for 71.1%
Don't know for certain, but from memory . . .
I-39: 38/47 (81%)
I-41: 38/92 (41%)
I-43: 30/70 (43%)
I-90: 28/41 (68%)
I-94: 48/101 (48%)
So overall, 182/351, 52%.