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A road sign is more easily observed and identified and its message perceived in advance and eventually read if different classes of signs conform to different shape, size and colour codes.
The shape and size of a road sign play an important part in ensuring that drivers actually see the sign. This function is termed the "attention conspicuity" of the sign, and refers to the ability of a sign to attract attention when the driver's attention is not specifically drawn to the probable occurrence of the sign.
More important regulatory signs, those that are mandatory and prohibitory, are normally circular, although there are exceptions, such as the octagonal stop sign, the triangular yield sign, and the diamond-shaped pedestrian priority sign. All other regulatory signs are rectangular, for instance the one-way roadway sign, and all reservation and comprehensive signs. Signs that warn of a hazard ahead are, with a few exceptions, triangular in shape.
Guidance signs are generally rectangular. when the message is brief, rectangular signs are also small. However, those used for more complex guidance or directional purposes are relatively large.
The different colours used on differently-shaped signs contribute to the early comprehension of the sign class, or the level of the sign within the overall sign hierarchy.
Virtually all road signs may be used in either a permanent or temporary form. The main exceptions are regulatory control, location and tourist information sings. Permanent signs include all road signs used to indicate normal conditions - the status quo. A selective restriction regulatory sign which, for instance, limits the applicability of a regulation to only a part of every day, remains classified as a permanent sign because its application, although only for part of the day, is constant.
A temporary sign with a yellow background colour is any sign that is used to indicate a change in normal circumstances, whether this be for a few minutes, hours or for several months. The message that should be inferred by an observer of temporary signs is that normal standards may not apply, calling for a higher level of attention to surrounding circumstances.
Typical temporary signing circumstances include:
The basic shapes and colour codes for the various sign classes are shown in the table on the next page.