It would! As you say, roads (or cycleways) beside motorways are usually short, have little traffic, gentle gradients and often a prized position in a valley rat

It would! As you say, roads (or cycleways) beside motorways are usually short, have little traffic, gentle gradients and often a prized position in a valley rather than lumbering up a hillside. Ideal cycle routes... apart from the motorway a few metres (/feet) away. I've got a page of notes on possible ways of fixing this - it is a hard problem. One possibility I'm toying with is a massive grid of "scenic value" for (say) each 0.01° x 0.01° square, so I could downrate the surroundings of motorways and uprate routes through (say) national parks.