A mandatory sign is a subset of the regulatory sign group and must have at least 60 centimetres across on rural roads, they are used to set the obligations of all traffic that uses a specific area of road and most road signs are circular
Mandatory signs are a subset of the regulatory sign group as defined by the United Nations Economic and Social Council in the Vienna Convention on Road Signs and Signals of 1968, and are often seen not just on their own, but used in conjunction with other signs, traffic lights and bollards as a form of visual shorthand within these objects. The sign must be at least 60 centimetres (24 in) across on rural roads, or 40 centimetres (16 in) in built-up areas, although mandatory signs incorporated in traffic lights, bollards or larger road signs can be as little as 30 centimetres (12 in) in diameter. Mandatory signs are road signs that are used to set the obligations of all traffic that uses a specific area of road. In Vienna Convention signatories, the mandatory sign is either a light blue circle with a white border (Type A) or a white circle with a red border (Type B). 
