Those who have responded to my post and have sited specific instances or issues and problems using the interstates seem to think that you can extrapolate one incident and arrive at a conclusion
These same problems of narrow bridges with no shoulders, bad traffic, speeding traffic etc etc happen on back roads and I would suggest are orders of magnitude more frequent and more serious than can be experienced on the interstates.
"...extrapolate one incident and arrive at a conclusion?" Is that kettle black?
I think you're overstating the case drastically with "orders of magnitude." Try riding an interstate in, say, rural Illinois or Tennessee, even though it's illegal there, and tell me how often you are safer on the interstate shoulder. There are pleasant surface roads through there, by the way.
I've driven interstates in about half of the states, and the 10 mile stretch on the TransAm going into Sinclair, WY was the anomaly in my experience. It was uncommonly good for cycling. Straight road, wide shoulders. Even as good as it was, it wasn't terribly comfortable being passed by trucks that didn't move into the left lane.
Speeding traffic on a back road may be driving 50 mph. On an interstate it'll be 80-90. That's three times as bad if you get hit. (Square the velocity!)
When you run into a stretch of road or a bridge with no shoulders, the traffic has a better chance to see you and slow down and/or move over at the lower speed.
You really haven't answered the point about entrance and exit ramps.
You really haven't answered the point about (demonstrably) orders of magnitude higher traffic count on interstates.
As unexpected as a bicyclist is on a surface street, they're much less expected on an interstate, so the passing drivers will take longer to perceive a cyclist
I can't agree with your conclusion. We may have to agree to disagree.