How broad is your definition of 'interstates'? Just the ones designated with an 'I'? How about 'US' and state numbered roads that are divided (4 lane) without or without limited access? How about bypasses around towns?
What makes a route 'safe'. Statistically limited access divided highways are the safest. Many accidents are the result of cross traffic, and passing traffic. And shoulders are broader and less cluttered in case you 'decide' to deviate from your designated lane. :)
But if you just want to take your time and see the farms and towns, there are many choices. You don't have to stick with one route number. Just be ware that in states like Iowa, most roads run EW or NW. There are few diagonals to 'cut off the corners'.
