<P> Mandatory signs are road signs which are used to set the obligations of all traffic which use a specific area of road . Unlike prohibitory signs, mandatory signs tell traffic what it must do, rather than must not do . Most mandatory road signs are circular, may use white symbols on a blue background with white border or black symbols on a white background with a red border, although the latter is also associated with prohibitory signs . </P> <P> 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 . 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). The sign must be at least 60 cm (1.96 ft) across on rural roads, or 40 cm (1.31 ft) in built - up areas, although mandatory signs incorporated in traffic lights, bollards or larger road signs can be as little as 30 cm (0.98 ft) in diameter . </P>

What is the shape of mandatory traffic signs