It's a sign !2014-09-21 (571)
90km north of Upington there was a Sociable Weaver colony in a tree next to the old untarred road, as seen in the Google Streetview photo. This tree is very close to the 90km road sign and in 2010 the back of the sign had no nest material. By 2012 some branches of the tree had collapsed and some Sociable Weavers built a nest on the back of the road sign. In 2014 there does not seem to be any change.
There is no other record of a weaver nest on a road sign (to my knowledge)! (click on thumb-nail to see full record)