Mandatory signs are subject to regulation and used in conjunction with other signs. The mandatory sign for Road signs has standard sizes and diameters. Other mandatory signs for indoor and outdoor use have other standard sizes.

Mandatory signs can also be used to issue instructions to all vehicles - common examples include "pass on this side" signs seen at roadworks, and "compulsory roundabout" signs seen at mini-roundabouts.

Other signs of the type could include "compulsory direction for vehicles carrying dangerous loads", used to divert vehicles carrying explosives or poisonous chemicals away from areas with open flames such as oil refineries. Minimum speed limits can also be defined using mandatory road signs.