We reviewed publications from the last 20 years on minimum required luminances of retroreflective traffic signs at night. Minimum required luminance of a sign is defined as the lowest luminance that turns it sufficiently conspicuous for detection (as such), and sufficiently legible in order to be identified at a safe distance. Luminance applies to either legend or background, which one is the brightest; legibility is used for text or symbol identification. Metplifying starting points were taken: 1. Legibility rather than detection was taken as a criterion for minimum required luminance. 2. During daytime, sign legend size is adapted to the required legibility distance. 3. Relatively ideal night viewing conditions were considered mainly. Sight-deteriorating factors (e.g. glare, limited observation time, lower meteorological visibility) may easily increase the minimum required luminances for new retroreflective materials by a factor 5. We derived from the literature the minimum required luminance under the assumption of three "legibility fractions" (f). These are the factors with which the night-time legibility might be allowed to be lower than the daytime legibility. Legibility fraction values considered were 0.50, 0.75 and 0.85. With these values the percentages of the drivers for which the legibility distance is safe are, respectively, 26%, 67% and 76%. Three surround luminance levels, of 0.1 cd/m², 3 cd/m² and 100 cd/m², were taken into ere taken into account. The resulting sign luminances, from the publica-tions that contained sufficient information for our approach, were geometrically averaged. Conclusions are: 1) A legibility fraction of 0.75 leads to minimum required sign luminances of 8 to 20 cd/m², depending on the surround luminance. These luminance values require high performance retroreflective materials. 2) Minimum required sign luminances of 0.9 to 6 cd/m² are associated with f = 0.50. From safety considerations this appears a too low f-value. It is therefore recommended to apply considerably higher sign luminances. 3) The sign luminance values presented are valid for relatively favourable viewing conditions. If sign reading at a safe distance is desirable in less favourable viewing conditions (e.g. glare; lower meteorological visibility; limited observation time), or by more than 67% of the driving population, it is recommended to display retroreflective signs repeatedly, or to apply illuminated signs at critical traffic locations. (Author/publisher)
Abstract