What's the traffic like on I-80? If it's light, get a mirror, ride centerish, and shift as necessary to let traffic by (if you even need to shift). If it's heavy traffic in both directions and you don't feel comfortable being out in the lane with the traffic (speed limit?), then I'd use a different route. A 2 foot shoulder is not wide enough to safely ride on if there's a drop off to your right.
Gravel roads wouldn't be bad on the right bike but your speed will certainly suffer. What type of bike do you currently ride?