Legibility of road marking symbols in the roadway

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Göran Nilsson

The regulations concerning road markings have recently opened up more for use of symbols in the roadway. However, regulations for the dimension of road marking symbols vary even between the Nordic countries, and research on how the symbols should be designed is lacking. The aim of the ViP project described in this report was to establish guidelines for how symbols in the roadway should be extended along the road. In order to achieve the goal, a simulator study on legibility distances was carried out, followed by two minor field studies. In the simulator study, four two-character symbols and four three-character symbols were used; 55, 56, 65, 66, 55N, 56N, 65S, 66S. For each of four symbol sizes; 1.6 m, 2.5 m, 4.0 m and 7.5 m and each of four speeds; 30, 50, 70 and 90 km/h, one two-character symbol and one three-character symbol were presented to the driver. The simulator study resulted in a main effect of symbol size; the larger the symbol size, the larger the legibility distance. Another result was that the legibility distance decreased when the speed increased. As an attempt to check the results from the simulator a downscaled field study was carried out. The highest and lowest speed and the largest and the shortest symbol size in the simulator were used, although downscaled with a factor three. Comparing the results of the simulator and the downscaled field study, the speed effect was significant but weak, i.e. the effect size was small. To be able to establish if there is a speed effect that should be examined in a larger test, a validation field study was completed. In the validation field study, the symbol size was always 2.5 m and the travel speeds were 30, 50, 70 and 90 km/h, in correspondence with the simulator speeds. The results of the validation field study showed no speed effect. The simulator study in this project confirmed the logic of larger symbol sizes meaning longer legibility distances, but while the speed influenced legibility distance in the simulator this was not the case in the field. The legibility distances in the simulator were also shorter than in the field, possibly due to issues related to the graphics rendering. A conclusion from this is that in order to find out how much road marking symbols should be extended, supplementary field studies are needed. It is recommended that future studies include several symbol sizes but it would probably be sufficient to perform the tests at only two vehicle speeds; one low and one high. Data from such studies should also be used for developing the graphics rendering in the simulation.

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