0898fffa_s__2_plus_1_roads__and_Signals__WHAT_THEY_MEAN
[ROAD MARKINGS] A single broken yellow line along the side of the road [WHAT THEY MEAN] 
This road contains a hard shoulder, which is normally only for pedestrians and cyclists. If a driver wants to allow a vehicle behind them to overtake, they may pull in to the hard shoulder briefly as long as no pedestrians or cyclists are already using it and no junctions or entrances are nearby. Different rules exist for hard shoulders on motorways. See Section 11 for details.
Traffic Signs, Roadway Markings, 2-plus-1 roads, and Signals
WHAT THEY MEAN