Two Way Traffic Post Mounted Sign - Dia 521 R2/RA2 (Face Only)Diagram 521 is a warning for the road changing from a length of dual carriageway to a single carriageway with two way traffic. This may be used with plates to show the distance two way traffic will begin, "Distance Ahead" diagram 572 or "For Distance" 570 to describe for how long to expect oncoming traffic. You'd find this sign coming off of a one way dual carriageway into single lane, two way traffic. It should be erected after the "End of Dual