Dear John Johnstone: the white bars are protection against tree branches and stone walls (like canyons) of north-east Portugal. At this point (Miranda do Douro), River Douro (the same of the Port Wine), has is own gate to come in Portugal, in a landscape like a stonethroat. In the hills, some bush specimens developes a thermic power near 800 degrees Celcius.
Either way, it looks the part and i wouldn't argue with it!
Added by John Johnstone on 11 February 2008.