Right first thing is to make sure the farmer owns the permission I guess that being a duel carraigeway t be the councils so due to this location there is fair chance that you could get a tug from the plod when somebody phones in some dodgy guys in camo rooting around the hedge .So there could be your first issue
But if all is well on that then roadside ferreting can be the most productive ferretting as in my time have learnt that its very very rare from the warrens to go below the tarmac edge right upto it yes but rarely under due to make up of the roads gravel sub bases coneys love the