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 Vetus Salina, the Roman name of the fort in Adony, Hungary
Notice the two lines representing the Danube River in the right side of the picture.
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