What do such traffic signs indicate?

This blue sign with a white number is the identification sign for an autobahn (motorway) in Germany. It shows the specific number of the autobahn you are on, in this case, autobahn 48. This helps drivers know exactly which motorway route they are using, which is especially important for navigation and orientation on the German highway system. It does not indicate distances or petrol stations, nor does it refer to a European road number. Remember, these signs are always blue with white numbers for autobahns.