Plaques on the ground in Budapest and marking the line of fortresses, camps and stations of the Roman army along the Danube (Pannonian Limes)
The current plaque shows Ad Novas, the Roman name of the fort in Zmajevac, Croatia
Notice the two lines representing the Danube River at the right of the plate.
In the next photo taken on the same day, part of the line of fortresses was seen, with the route of the Danube River between the various plates Click for a larger image
More details can be found here Click for the item in The ultimate street signs site