I have worked on the roads, in one way or another, for 25 odd years. There is no doubt that frost damage is the main factor, but companies/agencies have been trying out new surfacing techniques and I think this contributes to the failure of the surface. When I first started out we used HRA(Hot Rolled Asphalt) with pre coated chips rolled in. I can't remember the last time I saw a chipping machine as everything now seems to be SMA (Stone Mastic Asphalt). The advantage of SMA is its quicker to lay, because you can just roll it as soon as it comes out of the paver, and its a quieter road surface. However, I don't think it lasts as long as there are sections I know that have been replaced after less than 2 years! As to motorways being in better condition, they aren't really...its just that at 70 mph you are going too fast to notice. I recently did the M1 between J18 and J16 and whilst I would have said it wasn't too bad, when you actually got to stand on the motorway you could see big chunks of missing wearing course and lots of big cracks.