Map-type advance trail blazer

GE11

Where: On the left-hand side of any road where two routes join the numbered route by means of a set of one-way carriageways.
Purpose: To indicate that a route of equal or higher status can be entered from the road on which you are travelling. This new route consists of a pair of one-way roadways.
Action: You need to know before hand in which direction you want to travel, e.g. North(N) or South(S).

Manual on Road Traffic Signs, page 41.