The example you use, I agree i may use the Dual carriageway, but would have to accept there are other risks. But, how about a Single carriageway A road, mile long stretch which is used by lorries, vans, cars, is fairly poorly lit and has a double width pavement split for pedestrians and cyclists. Pavement set back 20 foot away from the A road. Very rarely any pedestrians or dog walkers as not near housing. Which would you use?
Every situation is different, but the default should be to use the facilities that have been provided to improve safety.