Origamist said:As someone who spent quite a few hours on the A38 also known as the Devon Expressway (70mph limit) I know where you're coming from. That said, I would not want to downplay the problem of sliproads and traffic moving at between 70mph and 90mph+.
Whilst sight lines are often better on dual carriageways, motorists tend not to see cyclists on trunk roads and this does not help matters when they do encounter cyclists riding at 20mph or so. It's true that oncoming traffic is not a problem, but with a stream of overtaking traffic in the oustide lane you need to be constantly aware of what's going on behind you due to the huge speed differentials.
It's quite interesting because although there is a risk on the slip roads on quite a few high speed dual carriageways often the ones that don't have slip road have large waves and clumps of traffic coming up behind you in both lanes as somewhere further back there was a bottleneck (probably light controlled).