VISIBILITY
1.10 For road markings to be effective, they must be clearly visible both by day and by night. Markings have two principal functions. The first is symbolic, e.g. hatched markings; the driver needs to have learned that these indicate an area which is not available to traffic. The second is guidance; centrelines, edge lines and lane lines help drivers to maintain their lateral position on the road. Some markings, e.g. hazard lines and double white line systems have both symbolic and guidance functions.
1.11 The guidance function is less critical (although still important) in daylight or on lit roads because there are many visual cues available to enable the driver to judge course and position. On unlit roads at night, conditions are very different; the visual stimuli in the distance and to the sides of the road are largely absent. Road markings then become the most important aid in enabling the driver to follow the road.
1.12 Recent collaborative European research has shown that drivers need to be able to detect guidance markings at a distance equivalent to a minimum of two seconds of travel time. If the visibility is less than this, drivers tend to adjust too late when the road changes direction. They run too close to the centre line on left hand bends, or too close to the road edge on right hand bends. The higher the prevailing traffic speed, the greater the visibility distance required to maintain this two-second "preview time". If it is not provided, drivers tend to miss the curve, or proceed in a series of staggers.
1.13 A variety of factors influence the visibility distance of a road marking. It is increased when a line is wider, has a higher mark-to-gap ratio or has a higher coefficient of retroreflected luminance (in the day time, higher contrast with the road surface). Visibility distance is adversely affected by glare from oncoming vehicles, dirty headlamps or windscreen and especially by rain; the glass beads which produce the night time luminance are drowned by excess water, greatly reducing the brightness of the line. Older drivers also see a marking less well than the young; someone seventy years old may suffer a reduction in visibility distance of more than 20% compared with drivers still in their twenties.
1.14 The marking regime prescribed in the UK generally gives adequate levels of guidance in good conditions, i.e. where the road is dry, the driver is young, the vehicle has clean, powerful headlamps and there is no glare from oncoming vehicles. On roads with high traffic speeds, wider lines should normally be adopted where alternatives are prescribed. However, it is important that guidance markings are well maintained. Severe wear reduces both effective width and retroreflective performance, and hence the visibility distance. Further guidance on the maintenance of road markings can be found in paras 23.21 to 23.26.
REFLECTORISATION
1.15 Tiny glass beads are incorporated in road markings so that they reflect the light from vehicle headlamps back towards the driver. This makes the marking much brighter at night than non-reflectorised materials. The new European Standard for road markings (BS EN 1436) specifies several different classes for night-time brightness. Brighter markings are visible at greater distances, and may provide an acceptable level of performance for a longer time before renewal becomes necessary (see paras 23.9,23.10 and 3.16 for further details).
1.16 Markings which maintain night-time performance even when wet may also be specified. This is usually achieved by the use of larger glass beads, but the wet performance of certain road markings may also be enhanced by the use of raised profiles (see paras 4.39 to 4.48, and 23.16).
DIMENSIONS
1.17 Dimensions on the figures are in millimetres unless stated otherwise. Many markings are fully dimensioned in the Regulations. Detailed drawings of the more complex ones are published by the Stationery Office in the series "Working Drawings for Traffic Sign Design and Manufacture" and also on the Department's website.
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