I drove a 1969 Subaru 360 cross country, and found out to my surprise that it was a lot safer to go on the interstate than it was to go on the highways.
1: There's pretty much always a passing lane, so traffic doesn't get crowded behind you, and drivers act less like assholes when they can get around at any point, and you never have to pull over to let traffic pass.
2: There are fewer grades, and the ones that do exist are usually easier than those on highways, so you go faster.
3: The minimum speed on the interstate in most states is 45 or 50mph. On highways it is usually 55.
The Subaru 360 has a cruising speed of 50mph, and top speed just shy of 70... I tried the highways and always got a line of cars behind me at every hill and had to pull over and let people pass. I got pulled over too for it.
After that I stuck to the interstate and didn't have a single problem for the rest of my trip.