(a) the lateral clearance between the sign and the edge of carriageway and,
(b) the length of time needed for reading and absorbing the message. Drivers should not have to divert their eyes more than ten degrees away from the road ahead. This means that the message on a sign must be fully absorbed before a driver reaches the point where the observation angle exceeds ten degrees. If, as may be assumed, a driver needs a certain time to absorb a sign’s message, the faster the speed of approach the further away must the reading of the sign commence. Reading must be completed and the message absorbed before reaching the point where concentration on a sign would distract attention too far from the road ahead.
1.32 These considerations have led to the design of signs of different sizes to suit different speed values. For directional and informatory signs where the legibility of the words is most important different sizes of alphabet are used. For ideographic and symbolic signs the size of the sign is proportional directly to the approach speed of traffic. In subsequent chapters details of these different sizes are given.
1.33 The lettering chosen for nearly all road signs is lowercase with initial capitals. There is one alphabet for use with light lettering on a dark background and a second for dark lettering on a light background. A range of numerals, separators and other characters is also available for each alphabet.
1.34 There is additionally a special range for the route numbers on motorway signs.
More details of these alphabets are given in Chapter 7.
1.35 In addition to distinctive shapes, different classes of signs have distinctive colour combinations. The number of different colours which can be usefully used on signs is limited by both aesthetic and technical requirements; Appendix 1 lists the colours with their specifications. Subsequent chapters describe their use in detail.
1.36 With standardisation of types of signs, there must also be uniformity in the use, siting, mounting height, illumination and reflectorisation of signs within limitations imposed by site conditions.
1.37 As already emphasised uniformity in the use of signs is of first importance and is dealt with in detail in later chapters.
1.38 The Traffic Signs Regulations and General Directions limit the use of certain signs and certain classes of signs. The Traffic Signs Manual not only explains in non-legal language the requirements of the Regulations and Directions but also advises on all aspects of signs and their uses including matters not covered in the Statutory Instrument. If all highway authorities follow this advice a reliable and uniform system of traffic signing will be ensured and this will benefit road users throughout the United Kingdom.
