They are countdown markers to the next exit
The exit from a motorway is indicated by countdown markers. These are positioned 90 metres (100 yards) apart, the first being 270 metres (300 yards) from the start of the slip road. Move into the left-hand lane well before you reach the start of the slip road.
B They are countdown markers to a bridge
C They are distance markers to the next telephone
D They warn of a police control ahead