2DESIGN RULES COMMON TO ALL RECTANGULAR SIGNS
ALPHABETS
2.1 The alphanumeric characters used on traffic signs are from a specially designed alphabet known as the Transport alphabet. There are two versions: Transport Medium for white characters on a green, blue, brown, red or black background (Schedule 13 Part I in the Regulations); Transport Heavy for black characters on a white or yellow background (Schedule 13 Part II). Route numbers on green background signs are yellow and are from the Transport Medium alphabet. Some signs have an orange background and in most cases the characters are black from the Transport Heavy alphabet, but in diagrams 2714 and 2715 white Transport Medium characters are generally used. Transport Heavy characters use a slightly thicker stroke width than Transport Medium characters.
2.2 Light-coloured surfaces, especially when illuminated, irradiate into adjacent darker ones. Thus white characters on a dark background appear thicker than their actual size, whereas black characters on a light background appear thinner. The use of the medium alphabet for white and yellow legends, and the heavy alphabet for black legends compensates for this effect and ensures optimum legibility.
2.3 Most route numbers on motorway signs are from an enlarged Motorway alphabet. Again there are two versions: the standard Motorway alphabet for white characters on permanent blue background signs (Schedule 13 Part III in the Regulations); and the Motorway Black alphabet for black characters on temporary yellow background signs (Schedule 13 Part IV).
2.4 The four alphabets are shown on drawings TM 1, TM 2, TM 3 (TM being Transport Medium), TH 1, TH 2, TH 3 (TH being Transport Heavy), MW 1 (MW being Motorway White) and MB 1 (MB being Motorway Black). These form part of the series of working drawings available at www.gov.uk/ working-drawings-for-traffic-signs (see para 1.6).
TILES, X-HEIGHTS AND STROKE WIDTHS
2.5 To ensure correct letter spacing when forming a word, the characters in each alphabet are placed on imaginary tiles. The tiles vary in width, according to the size of the character, and have a fixed height which ensures correct line spacing. All design spaces are measured to the edge of the tiles and not to the actual characters, unless special rules state otherwise. Tile outlines must not appear on the finished sign.
2.6 The size of an alphabet is specified in terms of its x—height. This is the height of the lower case letter and is the same for both the Transport Medium and Heavy alphabets. The unit of measurement when designing a sign is the stroke width (sw) which is one quarter of the x-height and is not necessarily equivalent to the width of any given character. The dimensions shown in all figures in this chapter are given in stroke widths unless otherwise stated.
2.7 The tile height for any alphabet is twice the x-height (i.e. 8 sw). Thus for an x-height of 250 mm the tile height is 500 mm. For the two motorway alphabets, where there are no lower case letters, the units of measurement are still x-heights and stroke widths. Thus if the x-height of the main sign is 300 mm the tile height for both the Transport Medium and Motorway alphabets is 600 mm.
2.8 Figure 2-1 shows how the characters from the various alphabets are placed on the tiles. It can be seen that the lower case letters without ascenders or descenders are centred vertically on the tiles, leaving an equal gap of 2 sw top and bottom. The capital letters and numerals from the Transport alphabets are 5.6 sw high, with a gap to the top of the tile of 0.4 sw. The characters in the Motorway alphabet are 8 sw high and vertically fill the tile.
WORDS AND HORIZONTAL SPACING
2.9 Words are formed by butting the letter tiles together. The tile widths, listed in Appendix C, have been designed to ensure the correct spacing of the letters. However, for certain combinations of letters the tile widths have to be adjusted and these special tile widths are also specified in Appendix C.
2.10 The spacing between two words on the same line is 2.5 sw. Some signs indicate distances (e.g. 100 yards) or time of day (e.g. 8.30am). Where abbreviations are used for the unit of measurement the normal word spacing of 2.5 sw is reduced. Where dates are abbreviated, such as "15 Sept" or "Feb 98", the spacing remains at 2.5 sw. Figure 2-2 shows the appropriate horizontal spacing between different elements of the sign and for abbreviated legends. Where two symbols are placed side by side
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