Climbing ivy accented by morning glory flowers, that's what!
We really don't have much information to offer here. Did the occupants of these houses one day decide to train some foliage to climb up the signposts? I can't imagine that the city engineering department, which normally takes care of the streets and signs, is the responsible party.
Regardless of who was responsible, the signposts themselves are admirably suited to the training of climbing plants, since they are lined with holes all the way up. Morning glory plants do seem to run wild in this neighborhood, so it's possible that this was an act of nature.
On the other hand, the greenery around this stop sign strongly suggests the attention of some loving hand.