Influence of pavement surface characteristics on nighttime visibility of objects

Vehicle headlights do not light enough of a roadway length for safe nighttime driving at higher driving speeds. Therefore, particularly on highways with high traffic levels, fixed roadway lighting enables safe nighttime driving conditions. Roadway lighting design has evolved over the years from the illumination method, which is based on the amount of light falling on the road surface, to the luminance- and visibility-based methods that are in use today. Visibility of an object on the roadway is directly related to the contrast between the object and its surroundings. In nighttime driving situations, the pavement acts as the background for most objects on the road. Therefore, reflectance characteristics of the pavement are important in visibility-based roadway lighting design processes. Currently, pavement reflectance characteristics are incorporated through four standard reflectance tables (r-tables) developed to represent portland cement concrete, open-graded asphalt concrete, seal coat, and dense friction coarse asphalt pavements. In this research, the computer program STV developed by M. E. Keck, which calculates pavement luminance and visibility level, was used for a sensitivity analysis to evaluate how the pavement type and the standard r-tables influence these parameters. The analysis was conducted for fixed roadway lighting situations without the influence of vehicle headlights. Results from the sensitivity analysis indicated that standard r-tables are not sufficient to model the whole spectrum of pavement surfaces encountered in practice. An analysis of pavement reflectance data collected by the Road and Transportation Association of Canada revealed that asphalt-based pavements tend to increase their specularity and brightness with age, whereas portland cement concrete pavements display a decreasing trend.