How about this: point furthest from any interstate, within the lower 48. Taking a "break" from studying for finals, I tried to see if I could figure this one out. There are a few possible spots, it also depends how you classify it. All these distances are "as the crow flies".
Furthest point (that I found) from any interstate lies on the US/Canadian Border in northern Montana approximately at N 48.999656, W 107.695634. About equidistant from I15 south of Sunburst MT and I94 at West Glendive, MT, at ~190 miles, give or take a mile. This is the furthest point from any interstate overall, though since it's right on an international boundary, some may think it doesn't count.
Another contender could be this point: N 47.479758, W 87.968488. 183 miles from I75 just east of Rudyard, MI.
There are many large regions lined by Interstates in the Great Plains and the West. The seemingly largest one of these areas is surrounded by I15, I5, and I80, and encompasses parts of UT, NV, and CA, yet surprisingly, the point within this area furthest from any of these interstates is in Nye County NV (N 38.137041, W 116.539078), and is only about 150 miles from I15 near Vegas.
This is all pretty rudimentary work done on Google Earth, and if somebody spent more time, I'm sure they could more accurately pinpoint the locations, but I've got to get back to studying! Finals are approaching!