I've cycled on a few of these but am sure there are many others that are now relatively quiet. These roads tend to be well engineered, thus removing the steepest gradients, are often wide enough for vehicles to easily pass cyclists without coming too close, quite often still have some of the facilities like cafes and shops that grew up along them when they were trunk routes, and are often very scenic. This is because in many cases they were the original roads, which followed natural alignments such as ridges or valleys, and thus give fantastic views. The road I know best is the A38 in Gloucestershire, which runs parallel to the M5, and usually has little more than some local traffic on it.
Do please post any recommendations of similar roads (or warnings of roads that may not be as quiet as we might suppose). As I said in the other thread, I have it in the back of my head to write a short book on these one day, but it will probably never progress further than most of the other books I think of writing!
