4. The Functions and Classification of Signs
1.23 Signs are used to control and guide traffic and to promote road safety. They should only be used where they can usefully serve these functions. Warning signs will not, for instance, promote road safety if used widely where there is no unusual degree of danger. On the other hand their omission where guidance, control or danger warrants the use of a sign is not in the best interests of road users.
1.24 The advice given in subsequent chapters of the manual should therefore be closely followed.
1.25 Apart from carriageway markings and temporary signs there are three main classes of road signs. Each class has its basic shape and as explained in later chapters the use of certain colours is restricted to particular classes of signs. The three classes are:
(i)Regulatory Signs
These include all signs which give notice of requirements, prohibitions or restrictions. They may be either mandatory or prohibitory.
Regulatory signs are basically circular in shape and may be supplemented by plates beneath them augmenting the message given by the sign.
(ii) Warning signs
These signs give warning of a hazard ahead. The design of most warning signs is based on an equilateral triangle having its apex uppermost. They are sometimes supplemented by rectangular plates giving additional information as may be necessary.
(iii) Informatory signs
These signs normally give road users information about the route and about places and facilities of particular value or interest. Most informatory signs are rectangular but direction signs usually have one end pointed.
5. The Design and Use of Signs
1.26 In order to perform the function for which it is intended a sign must be capable of transmitting its message clearly and at the right time to road users travelling at the normal speed for the road. To achieve this a sign must have correct legibility distance, appropriate target value, simplicity of content and layout and effective illumination or reflectorisation. Signs must be adequate in design and construction without being extravagantly expensive.
1.27 The legibility of traffic signs is of prime importance. Its achievement depends mainly on the size of the lettering or the symbols used, although the use of adequate colour contrast between lettering and/or symbols and their background and the type of alphabet used are also important contributory factors.
1.28 Target value depends on both the colour and the size of the sign: a big sign will have adequate target value whatever its colour; but difficulties may arise with the smaller signs in urban areas in selecting sites with backgrounds which do not nullify the target value of the sign.
1.29 For simplicity of content and layout, ideographic representation of the message is most effective, but where lettering has to be used the message needs to be condensed into as few immediately comprehensible words as possible. Abstract symbolism is less satisfactory since its meaning must be learnt and remembered. In suggesting designs for signs to be specially authorised highway authorities should avoid abstract symbolism.
1.30 Size is the most important factor determining sign cost, therefore signs are designed to give the required legibility without wasting space.
1.31 Factors which determine the distance at which a sign should be legible at a given speed of travel are:
