What does a small square yellow sign with a red cross mean? I encountered this on the autobahn in austria, and it appeared directly before and after bridges. In addition, there was sometimes a similar sign with a white cross in the central reservation.
(from Google Street View, location)
These signs are, as some answers state, for snow plow operators to mark parts of the streets where the snow plow has to be lifted to prevent hooking into the surface and damaging the plow.
The crosses are placed on expansion gaps and other elevations in the road. There are also arrows which indicate to keep distance to the curb. Those are mostly placed on sewer grates and similar features on some side of the road.
As the snow plow usually moves relatively fast (above 50km/h) with a lot of downward force to scrape the ice from the surface, the damage while hooking into the street is colossal, not only destroying the plow, but also the truck.
In Switzerland, there are signs of the same size positioned at the beginning and end of bridges, overpasses, etc... along motorways and some secondary roads.
Likewise, there is no reference to them in driving education material, and they are different in each canton. In Valais and Bern cantons, they show the picture of a truck with a giant shovel or blade.
The commonality with Austria is the higher exposure to snowfalls, due to the geographic situation.
My guess about their usefulness would be: when the roads have to be cleared of snow in winter, the snowplow operators know when they are going to enter an overpass section. They may take extra caution not to push snow off the side that may fall on the other road below, and resume normal procedure after they leave the overpass section, marked by the second sign.
These signs are probably there for road maintenance crews, maybe to show where bridges begin and end. They aren't taught in driver's school in Austria.
