The paper presents a conventional approach to the assessment of highway visibility at night in foggy weather, based on the Visibility Level (VL) concept. The method is adapted from a nighttime visibility assessment method published earlier. First, we recall the basics of the VL concept and the associated target visibility computation model. We also recall the conventional obstacle collision scenario which serves to estimate the nighttime visibility distance of a small target ahead of the vehicle (e.g. a tire tread) as a function of pavement retroreflectivity for a given headlamp beam pattern, using the basic laws of photometry. Then we introduce the visibility impairing effects of fog. The first effect is the exponential attenuation of luminance with distance, described by Beer-Lambert law. The second effect is the veiling luminance generated by the backscattering of headlight, for which we propose an empirical model based on earlier results from physically-based simulations. We apply the presented approach to low-beam and high-beam patterns in different fog conditions, and we bring quantitative proof for the need to dip the headlamps in dense fog. Finally, we discuss the potential benefits of the presented approach for road operation as well as for automotive front-lighting and advanced driver aiding system design.
