The poor visibility of road markings in the dark and wet is a well known problem. In rainy weather, a reflective water surface is often formed on the surface of road markings, with the result that the light from the vehicle is reflected away from, instead of towards, the vehicle. This is the reason that conventional flat road markings can be hardly seen in the dark when it rains.
In a European project (COST 331) it has been stated that longitudinal road markings should be visible not less than 45 m in front of the vehicle on a road subject to a 90 km/h limit if driving is to be safe and comfortable. In wet conditions, a flat marking has a considerably shorter visibility distance and cannot be regarded to satisfy the requirements that road users can pose for safety and comfort. Can these requirements be met using road markings which have been specially designed to have good visiblity in dark and wet conditions?
In order to answer this question, a number of firms were invited to apply road markings, visible in the wet, on two test sections. No limits were imposed on the type of road marking to be laid, although the function was to focus on good visibility in the dark and wet.
The road markings were laid on the test sections in August 1998, and physical measurements of retroreflective properties (dry and wet), luminance coefficient and friction were made on four occasions up to May 2000. Further measurements of retroreflective property were also made on ten occasions.
Measurements on wet road markings showed that these can be made so that retroreflection initially exceeds 200 mcd/m2/lux. Function deteriorates over time, in such a way that for most materials it drops to about half this value after two winters. These results must be regarded very good, and can be compared with the requirements specified in the Nordic countries – 25-35 mcd/m2/lux.
A comparison with the results from COST 331, Chapter 5, shows that visibility in the dark and wet would initially have been ca 70 m for the best materials if they had been made as the "ordinary" Swedish edge marking, i.e. intermittent with a width of 0.10 m. Visibility deteriorated over a two year period to 55-60 m. This implies that they would satisfy the requirements for the absolutely shortest preview time according to COST 331. Whether they meet the requirements for the desired comfort is more doubtful. In this respect, COST 331, Chapter 6, states that 55 m is far too short a visibility distance if the speed limit is 90 km/h.
The results can thus be summarised as follows:
• Technically, it is possible to manufacture road markings that have considerably better retroreflective properties in the wet than those in the market at present.
• Even if these road markings, visible in the wet, are laid it is doubtful if the requirement concerning comfort for the conventional Swedish intermittent edge marking is satisfied.
If a visibility distance longer than 55 m is to be achieved in the wet, the edge marking must therefore have a greater aggregate area, i.e. it has to be wider or have closer spacing, or be continuous
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