Mandatory signs are subset of the regulatory sign group and must have at least 90 centimetres across 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.
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. 
