Light point distance and the control of the light intensity of dynamic road marking

The Dutch Ministry of Transport is considering the application of dynamic road marking (DRM) on motorways. DRM consists of a row of light sources (usually LEDs) built in the road surface. TNO Human Factors performed a psychophysical experiment to determine the necessary DRM luminous intensity for achieving adequate perception as a function of the light environment (time of day) and the geometry (distance between light points, orientation). The results provide a basis for the photometrical specifications of DRM. An experimental DRM was installed on an asphalt road surface on a closed terrain. At a viewing distance of about 100 m subjects had to adjust the luminous intensity of the DRM according to various perception criteria. The luminance of the road surface varied between 0.01 cd/m2 (night) and 5000 cd/m2 (sunny day). The DRM was oriented at a horizontal angle of 2.86 degrees with the viewing direction, similar to a traditional road marking at a motorway exit. The distance between light points varied 1 and 7 m. In order to establish the photometrical specifications for DRM the experimental results were combined with standard reflection properties of road surfaces, statistical data on daylight and other literature data. We derive a model that describes the required light intensity of DRM as a function of road luminance, light point distance, and perception criterion. For night- time road luminances (L 100 cd/m2) the required light intensity is proportional to L0.812. Differences between the perception criteria at daytime are smaller than at night-time. In daytime the maximum light intensity is a factor of 4.3 higher than the minimum light intensity; in night-time this factor is 14. The required luminous intensity of the individual light points increases with the (observed) light point distance, according to an asymptotic function. The minimum and maximum luminous intensities of the individual light points as a function of the road luminance can be calculated with the model. These intensity limits depend on the distance between the light points and on the orientation of the DRM. Calculation of the maximum required luminous intensity is based on road luminances of 4000 cd/m2 (95-percentile value) and 16000 cd/m2 (opposing sun). For a 2.86 degrees slanted DRM at a distance of 100 m, a mutual light point distance of 6 m and an opposing sun, the luminous intensity of the light points should be at least 100 cd. For a mutual light point distance of 1 m, the minimum value is 37 cd. The results of this study will be used by the Dutch Ministry of Transport for establishing a program of requirements for DRM