The visual task of a driver can be described by the visibility of a target in the field of view measuring approx.10 deg in angular size. Dunbar and de Boer conducted driving experiments in lighted streets using such targets and found, that we need a luminance difference between target and its background of DeltaL = 0.43 for safe perception at a road luminance level of approx 1 cd / m(exp2). This value can be confirmed by calculations which are based on laboratory measurements of a multitude of studies. They showed that DeltaL is dependent on: DeltaL = f ( L, alpha, t, contrast polarity of the target and age ) with: L road luminance, alpha = target size and t = fixation time The results, however, are valid for laboratory observation condition. For night time driving, a 'field factor' applies that accounts for the unexpectedness and the reduced fixation time of the target. This leads to a necessary DeltaL of 0.32, which is close to de Boer's and Dunbar's findings. The Visibility Level is indicated by VL = DeltaL / DeltaL threshold , the actual luminance difference divided by its laboratory threshold value. To perceive the form of the target, which is necessary in road traffic, higher VL's are required. Form perception is expressed by the visual acuity and has to be at least 0.66 for a person to obtain a driver's license in most countries. This requirement would lead to Visibility Levels of VL > 9 at road luminance levels of 1 cd / m(exp2) and even higher for lower road luminance levels or higher resolution demands. Perception deteriorates with age due to optical and retinal changes in the eye. It is shown that at the age of 60 years, the VL level drops to only 58 % from those experienced at 25 years.
Required visibility levels in road scenes at night time driving
Erfordernisse bezüglich der Sichtweite auf Strassen bei Nachtfahrt
2005
13 Seiten, 7 Bilder, 1 Tabelle, 9 Quellen
Conference paper
English
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