Finningley based JP5 nav training sortie back in the 80's. Having flown eastwards through the Lichfield corridor ATC helpfully gave us a cloudbreak to VMC below. Student (and I) didn't monitor our single DME and happily turned north to follow the A1 back to base. Only after a few miles did we realise that the road signs passing below were blue and not green - yep, we were following the M1 rather than the A1. No harm done but a good learning exercise all round.